1888.] ANATOMY OF THE MESOSUCHIA. 433 



The principal stumbling-block to the acceptance of the anterior 

 of the two ventral bars in the Crocodilian pelvis as os pubis would 

 seem to be its exclusion from the acetabulum. Should this con- 

 stitute an insuperable difficulty ? The os pubis is notably the more 

 variable of the three components of the pelvic girdle. Its ossifica- 

 tion is a later phylitic event than that of the ihum and ischium. 

 Not to refer to Labyrinthodonts, in which fuller information about 

 the pelvic girdle is still wanting, it is well known that in some 

 extant Amphibia — for instance, in Cryptobranchus japonicus and in 

 Salamandra maculatn — the ischium is well ossified, whilst the pubis is 

 still cartilaginous. This is so too in Rana esculenta ; and in Baty- 

 lethra capensis the osseous pubis is a small disk surrounded by 

 cartilage, whereas the iscliium is perfectly ossified. Even in higher 

 Vertebrates differences in the degree of development of the os pubis 

 occur, and this in nearly allied forms. Thus in the genus Lepus, 

 in L. timidus the pubis enters into the formation of the acetabulum ; 

 but not in L. cuniculus, in which, by dominant growth of the ilium 

 and ischium, the pubis is excluded from the acetabular cavity. Its 

 exclusion from this may also result from the great development of 

 a distinct ossicle (" os acetabidare "), which may become so large as 

 not to leave space for the pubis in the acetabulum. Of this, Talpa 

 europcea supplies an instance. Even in Homo an approach to this is 

 exceptionally to be found. Thus, in the Osteological section of the 

 Museum of the Royal College of Surgeons there is a skeleton of a 

 youth (Cat. No. 54 a, Ost. Series) in which both acetabula contain, 

 each, a large distinct ossicle of this kind, by which the area normally 

 occupied by the os pubis is much reduced, the areas contributed by 

 the ilium and ischium being much less encroached upon. Here we, 

 as it were, seize the pubis in process of being excluded. Does its 

 exclusion vitiate its claim to pubis 1 I submit that it has not this 

 force ; and, further, that the corresponding bone in Crocodilia, not- 

 withstanding that it has no share in the acetabulum, is also pubis ; 

 and this identification is in harmony with the fact that in the 

 embryo it forms with the ilium and ischium one continuous piece 

 of cartilage. 



Steneosaurus. 



Vertebral Column. — The plan of this in Steneosaurus being the 

 same as in Metriorhynchus, those details only will be noticed at 

 length in which they differ. 



Atlas. — The same elements similarly combined and without 

 evident formal differences are present. In aged individuals they 

 synostose, and the pars odontoidea synostoses with the epistropheus. 



Epistroj)heus (Plate XVIII. fig. G),— Reduction of its diapophysis, 

 the flatness of the lateral and the inferior surface of its centrum, 

 and the absence from this latter of the low keel or ridge, are the 

 most obvious differences. 



In vertebrae referable to the front of the neck behind the epistro- 

 pheus (fig. 3, p. 434), in which the parapophysis is placed very low, 

 the figure of the centrum nearly resembles that of the epistropheus. 



