Skull of Mochlorhinus platyceps. 165 



affairs long-continued usage justifies and establishes a title ; 

 and since nomenclature is a matter of convenience, it is a nice 

 point whether the unchallenged use of Owen's name for a long 

 time has given the reptile a preference over the crustacean. 



Professor Cope, in 1870, without apology, proposed the 

 name Lystrosaurus for a South-African fossil from which he 

 drew many of the characters of the skull in defining the 

 Dicynodontia. When this specimen was accurately figured 

 in 1892 it proved to be a typical example of Ptychognathus. 

 If that name should be eventually withdrawn from the South- 

 African fossil, Lystrosaurus is the only name which could 

 take its place, as Professor Cope has urged ; Zittel and others 

 have continued to use the name Ptychognathus. 



In 1889 Mr. Lydekker discarded Ptychognathus and adopted 

 in place Ptychosiagum, a new name applied by him to the 

 Indian fossil from the Panchet rocks in the Lower Gondwana 

 series, which Professor Huxley referred to Dicynodon, which 

 may be conveniently retained for that type. The Indian 

 specimens appear to differ in the shoulder-girdle, humerus, 

 vertebrge, and all known parts of the skeleton from the 

 remains of Ptychognathus^ which are only obtained from the 

 Upper Karroo rocks of South Africa, while Dlcynodon is 

 only known from the Middle Karroo series. No entire skull 

 of the Indian genus is recorded, and there is no evidence that 

 it has the generic character of a fronto-nasal angle in the 

 skull which distinguishes Ptychognathus from Dicynodon. 



The determination of the generic name is important be- 

 cause there are some allies of Ptychognathus which appear to 

 show that it indicates a small family of South-African reptiles 

 distinguished by the way in which the brain is elevated 

 above the floor of the brain-case as it extends forward. This 

 difference from some types of Dicynodon is comparable to 

 that seen in modern crocodiles, in which the front of the brain 

 is supported upon a median plate, while in Teleosaurs it rests 

 upon the basicranial bones. 



The subgeneric modifications which have come under ray 

 notice are two in number — first, a skull having a narrow 

 longitudinally channelled cerebral region, with outwardly 

 inclined sides, unlike the table-headed type of Lystrosaurus ; 

 and, although the straight face makes an angular bend with 

 the roof of the head, the surfaces are not parted by an angular 

 ridge, but round into each other. That genus may be named 

 Rhahdotocephalus J with the type R. Maccaigi. A second sub- 

 generic form named Mochlorhinus is distinguished from the 

 type Lystrosaurus by the perfect rounding of the face into 

 the flat table-top of the skull, coupled with the development 



