418 PROF. OWEN ON AN ANOMODONT EEPTILE PROM 



outer surface or dorsum near the anterior costa. The resemblance 

 of the scapula of the Platypus to that of Platypodosauriis, in general 

 form, is " increased by the origin of the spine close to the anterior 

 costa, and by the spine being bent forwards so as to seem to form 

 a continuation of the external surface of the scapula " *. 



The external surface of the scapula of Platypodosauriis (PI. XVII. 

 iig. 1) is flat in the proximal or basal half (« to h). and is here marked 

 by longitudinal striae ; it becomes convex, both lengthwise and across, 

 at the distal or humeral half (6 to e'), save where this expands beyond 

 the acromion to form the coracoid articular surface, /, when it be- 

 comes concave both transversely and longitudinally. The hind 

 border of the scapula, c, describes a concavity along its distal two 

 thirds, its proximal third being straight, as in Bicynodon leoniceps'f. 



The inner surface (ib. fig. 2) is longitudinally concave, flattened 

 at the proximal expansion; at the distal one it is transversely 

 convex, swelling out to contribute to the humeral articular surface 

 (ib. d), concave where it extends upon the beginning of the expan- 

 sion for the coracoid joint. The acromion curves towards this 

 aspect, and terminates in a thin border (ib. e'), showing no sign of 

 an articular surface for an epicoracoid or clavicle, such as exists in 

 Dicynodon J ; in this respect the process resembles rather the 

 acromion in Kisticeplialus §. A clavicle or episternal may have been 

 attached by ligament to the free margin of the acromion in Platy- 

 jpodosaurus. 



The striated superficial markings on this and other bones of 

 Platypodosauriis recall the character of those in some Labyrintho- 

 donts. Practures of the dorsum scapulae, or convex outer surface, 

 expose a lamellar or stratified texture of the bone ; the surface of 

 such lamellae, exposed beneath a superficial compact layer a line in 

 thickness, also shows, but more faintly, longitudinal strise. 



In Echidna (PL XYII. fig. 4) a ridge {g) is developed which repre- 

 sents the " scapular spine " of higher mammals, the second ridge and 

 process (e) answering to the second " spine" in Megatherium ; the latter 

 alone is present in Ornithorhynchus as in Platypodosaurus ; but the 

 process (e) is common to both monotremes as to the Triassic reptile ; 

 and the term " acromion " is here applied to it, as by Meckel || in his 

 Honotreme. 



. Humerus. — The humerus (PI. XYI. fig. 7) repeats the general pro- 

 portions of that oiPareiasaurus^,, with characters shown in the same 

 bone of Cynodraco ** and Dicynodon tt ; but its breadth in propor- 

 tion to the length is exaggerated by a special production of a tricipi- 

 tal process (PI. XYI. fig. 7, d) near the subsidence of the teretial 

 ridge (c'). This additional characteristic is not developed in the Cape 

 reptilian humeri above cited ; but it is present in that bone of the 



* Art. Monotremata, ' Cycl. of Anat.' yol. iii. (1847), p. 376, fig. 173, g. 



t Op. cit. pi. Ixx. fig. 1. X Ih. ib. e. 



§ Ib. pi. Ixix. figs. 8 & 9, 6. 



II OrnithorhyncM 'paradoxi Descriptio Anatomica, fol. 1826. 



*l[ Catalogue, ut supra (4to, 1876), p. 11, pi. xix. 



*'^ Ib. p. 19, pi. xxvii. tt lb. p. 43, pis. xH. &. xlii. 



