Liter-relationships of the Known Thcrocephalian Genera. 371 



of incisors present, and we are probably justified in assuming that 

 the forms with many incisors are ancestral to those with few. 



Alopecodon with 8 incisors and 2 canines might readily be 

 assumed to be the ancestor of all the later more normal types. 

 From such a type a i*eduction of the incisors to 5 large and 1 small 

 would give rise to the genera Scylacosaurus and Pardos7ichus, forms 

 which, though differing much in size and shape, agree in the 

 number of incisors and canines. In Glanosuchus the incisors are 

 still 5 large and 1 small, but the anterior canine is lost. A further 

 reduction of the incisors would give rise to such a type as 

 Scymnosaurus, where there are only 5 incisors, all large, and only 

 1 canine. In Ictidosatirus the little interior canine is retained, but 

 the incisors are reduced to 4 large and 1 small. 



Gynosuchus, Cynodraco, jElnrosaurns, Lycosaurus, and Tigrisuchus 

 are probably all further specialisations along this same line. 



By a second line Pristerognatmis may also have sprung from 

 Alopecodon. Here the incisors are 6, all large, and there is only 

 1 canine. 



A third line of descent appears to have given rise to some 

 genera which are characterised by having 2 large canines. Of 

 these Hycenasuclms has 6 large incisors, 2 lai'ge canine, and 4 

 molars. Trochosuchus has 5 large incisors, 2 large canines, and 

 3 molars ; while Lycosuckus has 5 large incisors, 2 large canines, 

 and only 1 molar. 



IctidosucJius and Arnognatlias appear to belong to a common 

 family which is not very nearly related to the better-known forms, 

 and Theriodesmus probably also belongs to the same family ; but 

 nothing further is known of their affinities. 



In the following diagram the relations of the better-known genera 

 are seen as far as the dental formulae are concerned : — 



Incisors. Canines. Molars. 



Alopecodon VVVVVV^^ vy WW WW 



Scylacosaurus VVVW^ vw VWWW 



Pardosuchus VVVW^ vw VWW 



Glanosuchus VVVW^ \J VWW 



Ictidosaurus WW^ vw VWVWW 



Scymnosaurus VWW W VW 



