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Show 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 constitute an insuperable difficulty 1 The os pubis is notably the more variable of the three components of the pelvic girdle. Its ossification is a later phylitic event than that of the ilium 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 maculata-the ischium is well ossified, whilst the pubis is still cartilaginous. This is so too in Rana esculenta ; and in Daty-lethra capensis the osseous pubis is a small disk surrounded by cartilage, whereas the ischium 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 acetabulare " ) , 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 ? I submit that it has not this force; and, further, that the corresponding bone in Crocodilia, notwithstanding 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. Epistropheus (Plate XVIII. fig. 6).-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 epistropheus (fig. 3, p. 434), in which the parapophysis is placed very low, the figure of the centrum nearly resembles that of the epistropheus. |