Horizon and locality: Mazon Creek shales, near Morris, Illinois.
The scapula of the present form is exceptional in its shape. It resembles an asymmetrical pyramid, the anterior side of the lower edge of the bone being contracted so that the anterior edge of the element is arcuate. Its top is very thin and possibly terminated in a broad cartilage. The lower end is thick and heavy and the articular surface is, apparently, well formed, though somewhat obscured.
The element identified as a clavicle is lying on its edge and has the proportions of the clavicle of Mazonerpeton costatum. The exterior edge is somewhat rounded and small. A portion of another element which I suppose to represent the coracoid lies alongside the humerus, although its form is quite obscured. ([Plate 3, fig. 3.])
The humerus has a remarkably well-formed head. In perfection of formation it corresponds well with that of the higher reptiles. This surface can even be divided into an anterior and a posterior articulation. The element projects posteriorly for the distance of 1 mm. from the surface of the shaft. The shaft immediately below the head is somewhat flattened and has an ovoid section. Further on it becomes flattened, a part of which is probably due to pressure during fossilization.
The elements of the forearm are both preserved and are approximately equal in size. They are remarkable in that they exceed the humerus in length, although they are not so heavy as that element. They are greatly elongate and slender, with the middle of the shaft only moderately contracted. The articular surfaces are well formed and both bones were hollow, as was also, apparently, the humerus. The ulna may be represented by the most posterior of the two elements, though the relations of the elements may have been reversed ([fig. 15, D]).
The base of the left wing of an orthopterous insect possibly allied to Paolia gurleyi Scudder lies between the radius and ulna. The nodule also contains impressions of plants, a portion of a frond of a Neuropteris and the impression of one of the Cordaites. Lying next the radius is a slender elongate element which may be a rib or a portion of a metacarpal. If a rib, it indicates that the animal belongs among the Branchiosauria. The fragment is only half as long as the radius and is entirely too obscure to base any conclusions. The other characters of the specimen point quite strongly to its microsaurian affinities.
The structure of the articular surfaces of the limb bones alone would indicate the microsaurian relationship of Erpetobrachium. It may be provisionally associated in the family Molgophidæ with such forms as Molgophis brevicostatus Cope, Molgophis (Pleuroptyx) clavatus Cope, and Molgophis macrurus Cope from the Coal Measures of Linton, Ohio.
| Measurements of the Type. | |
| mm. | |
| Length of scapula | 14 |
| Distal width | 6 |
| Proximal diameter | 3 |
| Length of clavicle (?) | 24 |
| Length of humerus | 25 |
| Length of ulna | 24 |
| Proximal width | 4 |
| Diameter of shaft | 2 |
| Distal width | 3 |
| Length of radius | 25 |
| Proximal width | 4 |
| Diameter of shaft | 3 |
| Width of distal end | 4 |