142 ANATOMY OF VERTEBRATES. 



amphibious reiitiles, when on land, rest the whole weight of the 

 abdomen directly ujoon the ground, the necessity of the modifi- 

 cation diminishing lial^ility to fracture further appears. These 

 analogies are important, as demonstrating that the general homo- 

 logy of the elements of a natural segment of the skeleton is not 

 affected or obscured by their subdivision for a special end. 

 The purposive modification of the hajmapophyses of the frontal 

 vertebra is but a repetition of that which affects the same 

 elements in the abdominal vertebraj. 



Passing next to the hromal arch of the parietal vertebra, fig. 

 93, H, ii, we are first struck by its small relative size. Its 

 restricted functions have not required it to grow in jjroportion 

 with the other arches, and it consequently retains much of its 

 embryonal dimensions. It consists of a ligamentous ' stylohyal,' 

 retaining the same primitive histological condition which obstructs 

 the ordinary recognition of the pleural element of the lumbar htemal 

 arches ; of a cartilaginous ' epihyal,' 39, intervening between this 

 and the ossified ha;mapophysis, or ceratohyal, 40 ; and of the ha;mal 

 spine, 41, which retains its cartilaginous state, like its homotypes, 

 in the aljdomen : there they get the special name of ' abdominal 

 sternum,' here of ' basihyal.' The liasdiyal has, however, coalesced 

 with the thyrohyals to form a broad cartilaginous plate, the anterior 

 border rising like a valve to close the fauces, and the posterior 

 angles extending beyond and sustaining the thyroid and other 

 parts of the larynx. The long bony ' ceratohyal ' and the com- 

 monly cartilaginous ' epihyal ' are suspended by the ligamentous 

 ' styloliyal ' to the back part of the tympanic at its junction ^vith 

 the paroccipital process ; the whole arch having, like the man- 

 dibular one, retrograded from the connection it presents in Fishes. 



This retrogradation is still more considerable in the succoedino- 

 hnsmal arch, fig 92, h i ; fig. 57, 5i. In comparing the occipital 

 segment of the Crocodile's skeleton with that of the Fish, fig. 81, 

 the chief modification that distinguishes that segment in the Cro- 

 codile is the apparent absence of its hasmal arch. We recognise, 

 however, the sjiccial homologues of the constituents of that arch 

 of the Fish's skeleton, fig. 34, in the bones 5i and 5-j of the Cro- 

 codile's skeleton, fig. 57 ; but the ui)per or suprascapular piece, so, 

 fig. 92, retains, in connection with the loss of its proximal or cranial 

 articulations, its cartilaginous state: the scapula, 5i, is ossified, as 

 is likewise the coracoid, 5l>, the lower end of which is sejiarated 

 from its fellow by the interposition of a median, svmmetrical, 

 partially ossified ])iece called ' episternum.' Tlic power of recoo-- 

 nisiiig the special homologies of so, 5i, and 5-> in the Crocodile. 



