364 ON THE SHOULDER-OIKDLE AND EXTREMITIES OF ERTOPS. 



cles are narrow, and expand but little at their contact with the episternum. They 

 form a symphysis behind and below the episternum, which has an exclusively anterior 

 and superior exposure. The latter is wedge-shaped in longitudinal section, thinning 

 out rapidly posteriorly, where it is not produced on the middle line as in the Clepsy- 

 dropidse. 



In Dimetrodon the form of the clavicle is similar to that of Crieotus. That is 

 the inferior poi-tion is bent inwards at right angles to the vertical portion, and is 

 greatly expanded in the anterior direction. Its inner margin is finely digitate by the 

 production of numerous radiating ridges. This portion of the shoulder-girdle projects 

 forwards in the manner of the so-called epiplastral bone of the j^lastron of the Testu- 

 dinata. With the episternum in place this part of the shoulder-girdle of Dimetro- 

 don resembles the anterior lobe of the plastron of a tortoise. A narroAving of the 

 prsecoracoid and scapula, and an elongation of the coracoid, would give the Testu- 

 dinate shoulder-girdle. Such a change in some unknown member of the Theromor- 

 ous order, together with modifications of the abdominal costoids, has given origin 

 to the order Testudinata. 



The anterior leg of Eryops is short, and very robust. The anterior foot has the 

 character, entirely exceptional in the Batrachia, of having five digits. The humerus 

 has been already partially described.* It resembles very nearly that described by 

 Professor Gaudiy to his Eucliirosaurus rochei.f It is short and very robust, and the 

 extremities are greatly expanded and almost at right angles to each other. The 

 proximal extremity with its band-like articular surface, is strongly decurved antero- 

 exteriorly, ending in a strong protuberance. The distal end has an enormous plate- 

 like expansion of the internal epicondyle, greater than that exhibited by 

 Euchirosaurus rochei. The condyle is subglobular and large, and the external 

 epicondyle is partly posterior to it. This epicondyle forms a process ' flat- 

 tened on the posterior face, which is continued upwards into the shaft 

 of the humerus, and is bounded below by a semicircular sharp edge. On the external 

 border of the shaft above the notch between the condyle and the external epicondjde, 

 is a short, obtuse, vei'tically compressed process directed at right angles to the shaft. 

 A similar process exists in EucMrosaurus rochei (Gaudry). 



The ulna has little or no olecranon, and has a strong glenoid cavity on the supe- 

 rior aspect for the condyle of the humerus. It resembles much that of a pelyco- 

 saurian. The shaft is slender, and the distal extremity is not expanded. At the 

 latter place the superior face is flat, and the inferior is convex. The radius is a short 



*Proc. Amer. Phil. Soc, 1880, p. 54. 



t Enchainements du Monde Animal ; Fossiles primaires, 1883, p. 277. 



