| Measurements of Pectoral Girdle of Specimen No. 4539, U. S. National Museum. | |
| mm. | |
| Width across the entire girdle | 17 |
| Length of interclavicle | 15 |
| Width of interclavicle | 10 |
| Length of clavicle | 11 |
| Width of clavicle, maximum | 6 |
Erpetosaurus acutirostris Moodie.
Moodie, Bull. Amer. Mus. Nat. Hist., XXVI, pp. 349-351, pl. lxi, fig. 1, 1909.
Type: Specimen No. 8598 G, American Museum of Natural History.
Horizon and locality: Linton, Ohio, Coal Measures.
The present species adds another form to the diversity of structure presented by the Carboniferous Microsauria. It is closely allied to Erpetosaurus (Tuditanus) obtusus Cope, from the same beds (Linton, Ohio), but differs from it especially in the position and shape of the orbits and the acute form of the skull. Other characters which amount almost to generic significance are found in the posterior prolongation of the frontal and in the triangular form of the skull. Only the skull of the animal is preserved. The character which is common to all members of the genus Erpetosaurus, the cranial rugosity, is present in this species on the squamosal and supratemporal. This character alone would not, however, suffice to separate the form generically, but the general morphology and arrangement of the cranial elements is such that reference to any other genus save Erpetosaurus would not be possible.
The skull of E. acutirostris takes the form of a rounded triangle. Its base is some 50 mm. in extent, and this width gradually narrows to 31 mm. across the orbits and still more towards the snout. The form of the skull is not widely different from that of the type species, E. radiatus Cope, but the differences are sufficiently apparent.
Nearly all the elements of the cranium can be detected ([fig. 22, D]). The bony portion of the cranium has nearly all been lost, leaving only the impression; and the matrix in which the skull was embedded has been forced up into the sutures between the cranial elements, thus forming ragged ridges where the bones of the skull joined.
The position of the nostrils can not be determined accurately. The orbits are placed well forward, a character common to several species of the genus. The interorbital space is equal to the long diameter of the eye. The orbits are separated by narrow prolongations of the postfrontals and by the anterior portion of the frontals. The frontals are remarkable in their great backward extension. In E. obtusus the frontals are nearly confined to the interorbital space. The parietals, which, on the median suture, inclose the parietal foramen, lie well posterior, and the parietals and the tabulare are small. A portion of the sculpturing of these elements has been preserved and it is seen to be made up of pits and elevations much as we find in the skull of Saurerpeton latithorax Cope. The left squamosal also shows sculpturing, which here tends to take the form of grooves and ridges, and also of pits and elevations. It is quite probable that the anterior portion of the skull was ornamented with grooves and ridges and undoubtedly the lateral-line canals were well developed. The postfrontals and the postorbitals are both large and elongated. The postorbital is especially large. The supratemporal apparently separates the tabulare and the squamosal in their posterior extremities. The squamosal projects posteriorly to the tabulare and apparently even goes beyond the limits of the exoccipital. The outlines of the jugal are fairly definite, as are also the limits of the maxilla.
| Measurements of the Type of the Skull of Erpetosaurus acutirostris Moodie. | |
| mm. | |
| Length of skull | 50 |
| Interorbital space | 9 |
| Width of orbit | 7 |
| Length of orbit | 10 |
| Width across orbits | 31 |
| Posterior width of skull | 50 |
| Diameter of pineal foramen | 1 |
| Length from tip of snout to posterior angle of skull | 65 |