584 Mr. D. M. S. Watson on some 



homologous. It remains to show that they correspond with 

 the bones which bear those names in Crocodilus, the standard 

 animal for reptilian nomenclature. No doubt arises as to any 

 of the bones except the splenial and coronoid. In all these 

 types except Dimetrodon the splenial enters the symphysis 

 and in some cases has a very large articulation with its 

 fellow. In Crocodilus the splenial does not enter the 

 symphysis, but in all Mesozoic crocodiles, in which there can 

 be no doubt of its identity, it has a symphysis, sometimes a 

 very large one. There is, in fact, no doubt that in all 

 primitive reptiles the splenial takes a large part in the 

 symphysis ; it does so, for example, in Cotylosauria {Paria- 

 saurvs, Limnoscelis, Diadeciis, Lahidosaurus, Captorhinus, 

 Procohphon)^ Therocephalia (all types examined), Cynodontia 

 (all types examined), Anomodontia (all types), Sauropterygia 

 {^Nothosaurus, Peloneustes, Pliosaurus), Ichthyosauria (^Ich- 

 thyosaurus, Oph(halmosaurus), " Rhynchoccphalia " [Rhyn- 

 chosaurus, Hyperodapedon ?, Champosaurus) , Thecodontia 

 [Belodon, Mystriosuchus), Crocodilia {Mystriosaurus, Steneo- 

 saurus, Metrtorhynchus) — a list which might be extended 

 indefinitely. 



The identity of the coronoid is not quite so obvious, owing 

 to the fact that it frequently extends far forward, in Dicyno- 

 don probably reaching the symphysis. In the living croco- 

 diles it is a very small bone forming the front of the supra- 

 Meckelian vacuity. In MetnorhyncJius, however, as has been 

 described by Desloiigchamps and as Dr. Andrews has demon- 

 strated it to me, it is a long bone stretching far forward 

 between the splenial and dentary, much as in Panasaurus. 

 In Peloneustes Dr. Andrews has shown that the coronoid, 

 which has exactly the same relations as in Metriorhynchus, 

 may extend forward to the symphysis, and does extend far 

 forward in all Plesiosaurs. 



These facts seem to me to justify the terminology of this 

 paper ; it is, however, interesting to examine a Stegocephalian 

 jaw in the light of them. The Stegocephalian mandible is 

 best known from the work of Branson on Anaschisma and 

 Eryops, checked and accepted for Eryops by Case, and 

 supported by Seeley's description of the hinder part of the 

 lower jaw of " LahyrintJiodon " lavisi'i and Smith Woodward's 

 of the mandible of " Lahy rintlwdon " leptognathus. 



These descriptions, when compared with the less well- 

 known mandibles of other types, go far to show that the plan 

 of structure is the same in all l^emnosjjondylus and Stereo- 

 spondyius Stegocephalia. 



Eothriceps is a small Teynnospondylus Stegocephalian of 



