ARTIODACTYLA.
Reading edition
GENERA INCERTÆ SEDIS.
ITHYGRAMMODON, gen. nov.
Our fragments of this fossil show an animal about the size of the llama, and approximating more closely to the Tylopoda than to any living forms.
The genus is established upon the two premaxillary bones, containing the incisors, parts of the maxillaries, the canine and the first premolar; besides fragmentary portions of the palatine plates. As the peculiar features of these remains render them unique, we are warranted in establishing the genus Ithygrammodon.
Generic characteristics.
The premaxillaries are long, narrow, and very straight, bent slightly in on their own axis as in Camelidæ, but expanded laterally at the end of symphysis as in Pecora. The upward processes are sharply defined, with a wide, rounded upper border.
The upper incisors are six in number, and are placed nearly in a straight line fore-and-aft, separated from each other and from the canine by small and subequal diastemas.
The incisors are nearly equal in size, cylindrical in shape, are placed vertically in their alveoli, and are slightly recurved.
The spine of the premaxillaries is long and thin, giving very long anterior palatine foramina, as in Ruminantia. The premaxillaries, in position, are close together, showing a very narrow muzzle.
The maxillaries have two well-developed canines, with long, laterally compressed fangs, recurved and becoming circular in section at the crowns. The diastema between the canine and the first premolar is proportionate to that in Procamelus.
Ithygrammodon cameloides, sp. nov.
Specific characteristics.
The premaxillaries (See Plate X., Figs. 1-4) are long, with a wide, strongly upward curving process; from the premaxillary symphysis to the third incisor, they increase in thickness; then narrow slightly to the maxillary symphysis. The upper margin of the process is smooth and narrow, curving very gradually upwards; while the lower margin runs more abruptly up, giving to it a strongly pointed curvature.
The outer surfaces are smooth and slightly convex; the inner are marked by a wide deep canal running from just behind the premaxillary symphysis into the maxilla.
The great size of this groove may perhaps be a generic characteristic; as it is much larger than in Protolabis (Plate X., Fig. 6), the modern camels, or any of the other ungulates, recent or fossil, that have come under our notice.
The premaxillaries do not co-ossify, as the spines are smooth on their inner side. Just outside the spines, and in front of the deep groove, the bones are marked by numerous small pits for the attachment of the muscles of the upper lip.
The portion of the maxilla in which the first premolar is situated, shows that the maxillaries swell considerably on their alveolar border; for the canine then narrow gradually for the first premolar diastema; then swell for its reception, and become very thin in the diastema between the first and second premolars.
The Teeth.—I. 3/?, C. 1/?, Pm. 1+/?, M. ?/?.
The incisors have quite deep fangs, are cylindrical in shape, vertical in position, and have no basal ridge.
The crown of the first is worn perfectly smooth, flat across the top, unlike the wearing exhibited in any modern form that has come under our notice. The others have their crowns broken, but indicate a nearly subequal series, with the last slightly the largest.
The canines have long, recurved compressed fangs, with rather short crowns, which are circular in section. They are much larger than in Procamelus or Protolabis of Pliocene, or the modern camels. (See X., Figs. 3, 5, 6, 7.)
The first premolar is the only one of the molar series preserved; its crown is chipped considerably, but nevertheless shows that it had one fang and no basal ridge. It resembles the incisors in form, but is slightly narrower and longer.
The genus Ithygrammodon has been placed for the present under the genera incertæ sedis; but its chief features point to an unmistakable affinity with the ruminants. It is probable that Ithygrammodon was the representative and the ancestor, in the eocene age, of that type of ungulates of which the camel and llama are the modern forms.
The line of descent of the camels was first indicated in the Proceedings of the Philadelphia Academy, 1875, p. 262; then in the Bulletin No. I, 1874, p. 25, of the U.S. Geological Surveys of the Territories, (Government Report of Explorations of 1873, pp. 498-500); and lastly in U. S. Geographical Surveys of Territories, vol. iv., pp. 341-44.
In the above writings, Professor Cope traces the development of the modern camels from the miocene genus Poëbrotherium, Leidy, showing the modifications found in Procamelus, Leidy, and Protolabis, Cope, of the succeeding age.
From the last work (Wheeler's Survey, vol. iv., p. 342), we will quote at length:
"The evolution of the existing types of Camelidæ is a good illustration of the operation of the laws of acceleration and retardation. In evidence of this we may follow the growth of the foot and dentition of the most specialized, and therefore the terminal genus of the series, the American Auchenia ----. It is well known since the time of Goodsir, that the embryos of ruminants exhibit a series of superior incisor teeth which disappear early. It is probable, but not certain, that in the miocene genus Poëbrotherium, as in various contemporary selenodont artiodactyla, the superior incisors persisted. I have, however, discovered that these teeth persisted in the Loup-Fork genus Protolabis during adult life. I have found that one (the second) of these teeth in Procamelus occidentalis persisted, without being protruded from the alveolus, until nearly adult age.
"In genera (e.g., the bunodont artiodactyla) where the incisors are normally developed, they appear at about the same time with the other teeth, and continue to develop to functional completeness. This development is retarded in Protolabis, since they are not so matured as to remain fixed throughout life in their alveoli.
"In Procamelus, the retardation is still greater, since the first incisor reaches very small dimensions, and is, with its alveolus, early removed; while the second incisor only grows large enough, and for a sufficient time, to occupy a shallow alveolus, without extending beyond it. In the first incisor the process of retardation has reached its necessary termination, i.e., atrophy or extinction; while in the existing Camelidæ the second incisor also has disappeared the same way. In ruminants other than Camelidæ, the third or external incisor has undergone the same process, while in the Bovidæ the canines also have been retarded in development, down to atrophy."
Professor Cope continuing the discussion of the teeth of these types, generalizes: "From these and many analogous cases, the general law may be deduced that identical modifications of structure, constituting evolution of types, have supervenea on distinct lines of descent?"
This summary of Professor Cope's conclusions has been necessary to show clearly in this series the probable place of the eocene genus.
From the shape of the premaxillaries and maxillaries, and the form and position of the teeth the camelline affinities have been indicated. Future research must be relied on to throw light upon the structure of the limbs and the axial skeleton; as it is from these that the conclusive proof must be deduced that Ithygrammodon is the ancestor of the camels, and that Poëbrotherium is the second link in the chain, instead of the progenitor.
Measurements of Ithygrammodon cameloides with Protolabis.
Ithygrammodon c. Protolabis.
Length of alveolar border of premaxillaries ·035 ·049
Greatest height of premaxillaries ·017 ·022
Greatest width of premaxillaries at last incisor ·013 ·01155
Thickness of premaxillary process ·010 ·017
Length of diastema between first and second incisors ·0035
Length of diastema between second and third incisors ·003
Length of diastema between third incisors and canine ·004
Teeth.
Fore-and-aft diameter of first incisor ·007
Fore-and-aft diameter of second incisor ·0075
Fore-and-aft diameter of third incisor ·008 ·011
Fore-and-aft diameter of canine ·012 ·012
Fore-and-aft diameter of first premolar ·011 ·011
Transverse diameter of first incisor ·007
Transverse diameter of second incisor ·0073
Transverse diameter of third incisor ·0085
Transverse diameter of canine ·0113 ·009
Transverse diameter of first premolar ·005 ·007