168 Structure, Organisation, fyc, of Fossil Reptilia. [Dec. 12, 



These osteological identifications are inconsistent with reference 

 of the type to the Labyrinthodontia. The author also examines the 

 relation of the Labyrinthodont type to existing Amphibia, and 

 regards the Labyrinthodont osteology as demonstrating closer rela- 

 tionship with Echthyosauria and Anomodontia. The group is there- 

 fore regarded as reptilian, forming a branchiate division of the class. 



The fossil is referred, to a new genus — Aristodesmus. It is identi- 

 fied as an Anomodont reptile chiefly on the basis of resemblances to 

 Procolophon and Pareiasaurus. It is shown not to be a mammal by 

 the large parietal foramen, the composite structure of the lower jaw, 

 and presence of the pre-frontal bone. It, however, differs from 

 known Anomodonts in making a somewhat closer approximation to 

 Monotreme mammals than has hitherto been evident, and this corre- 

 spondence extends to successive segments of both the fore and hind 

 limbs. 



The teeth are in sockets placed obliquely, with conical crowns 

 compressed to sharp lateral margins, and curved inward. The pro- 

 portions of the vertebral column are those of Echidna, though the 

 transverse processes are longer, as in Pareiasaurus. The ribs are 

 like those of a Monotreme, though the sacral ribs are longer. The 

 shoulder girdle resembles that of Procolophon, and differs from all 

 other Anomodonts in the constituent bones being unanchylosed, and 

 in the precoracoid having a large anterior extension in advance of 

 the scapula. The sternum appears to have been unossified as in 

 Crocodilia. The humerus is widely expanded at both extremities 

 and twisted, but does not show the peculiar lateral curvature seen in 

 Monotremes. The ulna gives no evidence of an olecranon process ; 

 it is larger than the radius, and appears to articulate with the 

 humerus. The pelvic bones are without acetabular or obturator per- 

 forations, are not anchylosed together, and the ilium is not expanded 

 transversely. The hind limb is ho larger than the fore limb. The 

 femur is more slender than the bone in Echidna. The fibula is pro- 

 longed proximally beyond the stout tibia, round which it may rotate. 

 The proximal row of the tarsus is one large bone, formed of the 

 blended astragalus and os calcis. 



In conclusion, the author argues that the points of structure are so 

 few in which Monotreme mammals make a closer approximation to 

 the higher mammals than is seen in this fossil and other Anomo- 

 dontia, that the Monotreme resemblances to fossil reptiles become 

 increased in importance. He believes that a group Theropsida might 

 be made to include Monotremata and Anomodontia, the principal 

 differences (other than those of the skull) being that monotremes 

 preserve the marsupial bones, the atlas vertebra, and certain cranial 

 sutures. Omithorhynchus shows pre-frontal and post-frontal bones, 

 and has the malar arch formed as in Anomodonts. 



