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Mr. E. S. Goodrich. 



not seem to be closely related to any particular mode of life or method of 

 progression, being essentially the same in reptiles of the most diverse habits. 

 Its development is possibly correlated with the formation of the mesotarsal 

 articulation so characteristic of the Sauropsidan reptiles and birds. It is also 

 accompanied by the disappearance of the fifth distal tarsal. Since there is no 

 reason whatever to suppose that it has arisen independently in the various 

 orders, we can only assume that the hook-shaped metatarsal was present in 

 the common ancestor of all the forms which possess it. The mesotarsal 

 articulation, above referred to, is due to the close connection or fusion of the 

 proximal tarsals with the tibia and fibula, and the distal tarsals with the 

 metatarsals. 



Now, in the Mammalia, the fifth metatarsal is of normal structure. When 

 the fifth digit is not reduced its metatarsal is straight, and articulates with 

 the cuboid (fourth and fifth distal tarsals fused) at the usual level. We 

 should not therefore expect to find a hook-shaped metatarsal in any fossil 

 reptile leading towards the Mammalia, and this expectation is fulfilled, since 

 there is no trace of it in the Theromorpha. 



Of the remaining Keptilia with one fossa and bar, the Ichthyosauria 

 unfortunately yield no certain evidence, since their foot is too modified. But 

 the more primitive Sauropterygia, such as the Lariosauridse, clearly display 

 a normal fifth metatarsal (fig. 3, F), thus confirming the view, which is now 

 gaining ground and based on other evidence, that the Sauropterygia are allied 

 to the Theromorpha. This view is, of course, incompatible with that of 

 Jaekel, who believes the Sauropterygia to be Diapsida which have lost the 

 lower temporal bar.* 



Also, if our contention is correct, that the modified fifth metatarsal is a 

 specialisation occurring only in that line of reptilian phylogeny leading 

 towards the Birds, we should expect to find it absent in all the Cotylosauria 

 and allied Protosaurian forms. Here, again, the facts support our view, for 

 these early reptiles have normal metatarsals like their Amphibian ancestors. 



It is clear, then, that we have here a valuable corroborative character to 

 help us to decide whether a given species belongs to the Theropsidan or the 

 Sauropsidan line of evolution. It will not be necessary in this paper to give 

 a description of the metatarsus of all known living and extinct Beptiles ; the 

 results of my investigations, based as far as possible on the examination of 

 actual specimens, but also to a great extent on the published figures and 

 descriptions of others, are summarised on page 274. But it is interesting to 

 consider briefly certain important genera and larger groups whose position 



* ' Zool. Anz.,' vol. 35, 1909. 



