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Show 422 MR. J. W. HULKE ON THE SKELETAL [Nov. 20, rugosus. In the former the 'intercalary* or intercentrum only is present; in the latter lizard it coexists with a genuine hypapophysis. In any comparison of the Crocodilian atlantal basilar piece with the foremost of the " subvertebral wedge-bones " of Ichthyosaurus, the morphological significance of the pair of long, slender hypaxonic styles attached to the former may not be ignored. These styliform bones were regarded by Cuvier as " apophyses transverses" (24). Their separate ossification is unfavourable to this view, which is not now maintained by anyone. Their inferior position might seem to suggest their being a form of chevron. Is this a tenable supposition ? The individual distinctness of each style, the absence of union of their ventral ends, is not sufficient, of itself, to refute this idea, since Ichthyosaurus and Plesiosaurus furnish familiar examples of the complete separateness of the two styles constituting their caudal chevrons. It is scarcely necessary to state that the reptilian caudal chevron originates in a downward extension of an intercentrum. This, as Dr. G. Baur has mentioned, is plainly demonstrable in Sphenodon (25). The development of the intercalated part seems often to be inversely proportioned to that of the freely ventrally dependent part that forms the chevron. The former may be reduced to a mere rudiment, or it m a y even disappear, whilst the latter m a y persist in its perfect form. I do not call to mind an example of the concurrence of an intercentrum and of a chevron, each being distinct, and both not forming a continuum. The pair of styles dependent from the posterior border of the basilar pieces do not, then, lend any support to the identification of the basilar piece of the Crocodilian arias with an (Ichthyosaurian) intercentrum. The obvious formal resemblance of the atlantal styles to the next posteriorly situated pair of similarly-shaped pieces, by all writers regarded as riblets, is a valid reason for regarding the styles also as ribiets. The chief and almost only difference is the simple form of their vertebral end, and their consequently single vertebral articulation. In estimating the value of this it should be borne in mind that the division of the vertebral end of the rib, which is so marked a feature in those of the other cervical vertebrae behind the epistropheus, is in Eusuchia usually indicated oidy by a shallow notch in the ribs of the vertebra just named. The ventral angle of the notch, which represents the capitulum costa?, is borne directly on a parapophysial facet or tubercle; whilst the upper angle of the notch, answering to the tuberculum costce, is commonly only connected by ligament with the diapophysis. From the rudimentary condition of the costal tubercle in the second pair of riblets, it is easy to conceive that a slight further reduction of it might cause its complete suppression in the first pair, and this appears actually to have occurred as regards the atlantal styles in the Eusuchia. Mesosuchia, however, retain a trace of a costal tubercular articulation in the little process which projects from the outer surface of the atlantal neurapophysis (cf. Plate XVIII. fig. J, d). The position of this little process in serial line with the upper transverse processes of the other cervical verte- |