( 379 ) 



6. — Note on the Specie^ 0/ Mesosaurus. — By R. Broom, D.Sc. 



There has hitherto been much difficulty ia differentiating the 

 species of Mesosaurus. The type of Mesosaurus tenuidens, and the 

 South African Museum specimen described by Seeley both show the 

 head and front half of the body, but both Seeley and I have hesitated 

 in referring the Cape specimen to the same species as the Paris one. 

 When, three years ago, I described the Nieuwondtville specimen, I 

 pointed out a number of features in which the two best known 

 specimens differed, but in view of the opinion of Gaudry, who had 

 examined the Paris specimen, hesitated in definitely declaring the 

 two to belong to different species. 



There has recently been presented to the South African Museum 

 an imperfect specimen of Mesosaurus from near Calvinia. It con- 

 sists of the cervical region and left fore limb, both well preserved, 

 with a very bad cast of the skull and of a number of ribs. On 

 comparing this specimen with Gervais' figure there can be little 

 doubt that the two belong to the same species, but there can also 

 be as little doubt that this species is quite distinct from the specimen 

 described by Seeley. In Mesosaurus tenuidens, as exemplified by the 

 Paris type and this Calvinia specimen, the head is about the same 

 length as the whole fore limb ; in the Capetown specimen the head 

 is apparently not longer than the humerus and radius together. In 

 M. tenuidens the back of the skull is more than 1| times as broad 

 as the radius is long ; in Seeley's specimen the skull is scarcely 

 broader than the radius is long. The cervical vertebrae in the type 

 species are well developed and mostly as broad as the distal end of 

 the humerus ; in the Capetown specimen the neck is slender, almost 

 all the vertebrae being narrower than the humerus. The specimen 

 described by Seeley must thus be regarded as a distinct species. 

 The question arises, however, whether it may not be the same as 

 the immature animal described by Giirich under the name Ditrocho- 

 saiirus capensis. Unfortunately Giirich's specimen represents only 

 the posterior region, with one slightly imperfect fore limb, and 



