94 SHOULDER GIRDLE OF CYNOGNATHUS chak 



pared to that " of Tliylacinus or Dissacus." Xo one can ex- 

 amine the actual sketches of the skull of that Theriodont 

 without endorsing that opinion. As a curious detailed point 

 of likeness to certain Mammalia may be mentioned " a small 

 descending process of the malar bone, which may be a diminu- 

 tive representative of the descending element of the malar 

 seen in EI other i urn, JVotothcrium, Dijyrotodon , Macroi^us, certain 

 Edentata, such as Glyptodon, Megatherium, Mylodon, Bradypus, 

 l)ut unparalleled so far as I am aware in fossil reptiles." (Osborn.) 

 The zoologist cannot help being impressed with the significance 

 of small details of similarity, which do not seem to l)e due in any 

 way to surrounding conditions of life, and thus referable to mere 

 convergence, like the fish-like form of Whales and Seals. 



The rest of the skeleton of the Theriodontia is by no means 

 so well known as the skull and teeth. But from what is known, 

 other mammalian characters can be pointed out. Perhaps the 

 most striking mammalian feature is to be found in the scapula 

 of Cynofinathus. It is in this creature somewhat narrow and 

 elongated ; but it has a well-marked spine, ending in a hooked 

 acromion. Now it is to be noted in support, so lar, of the 

 diphyletic origin of mammals, that in the Monotreme, as in 

 Whales indeed, the spine forms the anterior border of the scapula, 

 and is coincident with it, there being thus no prescapula at 

 all in the IMonotreme, and only a trace of it in certain Whales.^ 

 Whether the multituberculate Tritylodon or Diadcmodon had 

 a scapula after the Monotreme pattern is not known ; but it is 

 clear that the scapula of the triconodont Cynofinatlivs is quite after 

 the pattern of the Eutherian scapula. Furthermore, Professor 

 Seeley is of opinion that the coracoid was relatively small, and 

 indeed smaller than the same bone in Edentates, and a fortiori 

 than in Monotremes. Another fact of structure which points 

 also, possibly, in the direction of a diphyletic origin for the 

 Mammalia, is the double-headed ribs of Cynognathus. As is well 

 known, the ribs of the Monotremata have only the central head, 

 the capitulum. 



As a general mark of affinity with mammals the reduction of 

 the intercentra in Cynof/nathv.s may be noted, and also the 

 existence of a small though perfectly obvious obturator-foramen, 

 separating the pubis from the ischium. There are further details 



^ It may be uecessary to exclude the "Whales from the comparison. 



