422 MR. J. W. HULKE ON THE SKELETAL [NoV. 20, 



ruffosus. 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 sup- 

 position ? 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 inter- 

 centrum. This, as Dr. G. Baur has mentioned, is plainly demon- 

 strable 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 may even disappear, whilst the 

 latter may 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 atlas 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 riblets. 



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 Eusiichia usually indicated only by a shallow notch in the ribs of 

 the vertebra just named. The ventral angle of the notch, which 

 represents the capitulum costce, is borne directly on a parapophysial 

 facet or tubercle ; whilst the upper angle of the notch, answering to 

 the tnbercuhim 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 Eusuchin. 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. 1, d). The position of this little process in serial 

 line with the upper transverse processes of the other cervical verte- 



