OF DIDELPHYS VIRGINIANA. 51 



segment, as a whole, is not more unlike the succeeding one — the atlas — than the latter 

 is unlike the axis ; or than this last is different from a coccygeal, for instance. 



Aside from the displacement of its hcemal arch, its chief modifications relate to its firm 

 sutural (instead of loose articular) connection with the antecedent vertebra, and to the 

 expansion of its neural arch to accommodate the epencephalon. Taking away the ex- 

 panded neural spine, the three remaining elements form an osseous ring very similar in 

 general appearance to the atlas. It does not appear that the sum total of the modifica- 

 tions that this vertebra has undergone in becoming a part of the "skull" instead of the 

 " spine," is greater than that suffered by the contiguous cervical one, which latter is none 

 the less a " vertebra" for wanting centrum, neural spine and hajmal arch ; and it is cer- 

 tainly less than that characterizing the last coccygeal, — a bone that has nothing of a 

 vertebra but a degenerate centrum, fails to perform the essential office of a vei'tebra, and 

 is only recognized as a vertebra in virtue of its location, being to all purposes, as well as 

 appearances, the distal phalanx of a digit. Comparison of the two ends of the chain of 

 vertebral links shows that teleological modification is less obscure in the guise of hyper- 

 trophy, than under the mask of atrophy ; exaltation of the function of parts may entail 

 less change than degradation ; the caudal larval vertebra deviates more from the arche- 

 type than the cranial imago does. 



V. mesencephalica. Centrum and neurapophyses coalesce with the same elements of 

 the antecedent (frontal) segment; the compound bone so formed ("sphenoid") is simpler 

 than in some mammals, from not including an appendage of the maxillary arch ; for the 

 pterygoid is a distinct bone, as in birds. The neural spine is large, coincidently with 

 increase in size of the encephalon, though it is itself surpassed by the frontal spine. It is 

 developed from two centres, and the lateral moieties so long remain discrete, that they 

 are so found in the great majority of adult specimens; but they ultimately fuse together; 

 they never coossify with either frontal, squamosal, or sphenoid ; they reach their proper 

 neurapophyses. The parietal parapophyses (mastoid) are disconnected with that arch, 

 coossified with the acoustic capsule, and wedged in between the latter, the occipital neur- 

 and par-apophyses and the distal element of the appendage of the palato-maxillary arch, 

 which appendage serves as a strong osseous bridge connecting the hfemapophysis of the 

 nasal vertebra with the par- neur-apophysis of the ultimate and penultimate cranial seg- 

 ments, and slightly joins, also, the neural spine of the occipital segment. Besides the 

 extensive confluences and sutures that the parietal segment presents, it also contains — 

 i. e., is the site of — the frontal pleurapophysis, — the proximal element of the mandibular 

 arch (tympanic). This, however, has no osseous connection with the rest of the skuU ; 

 its relation to the parietal segment is merely one of position. The proper pleurapophysis 

 of the parietal segment (stylo-hyal) is wanting ; the other elements of the hyoidean arch ' 



'The OS hyoides may be conveniently noticed in this con- ported upon the b.isi-hyal, but also touch the thyro-hyals. 



nection. The size and general shape are shown in the The basi-hyal is small; as high as wiile; irregularly tive-sided, 



accompanying figure (fig. :>). The five or rather sub-quadrate, with a thickened, upright, posterior, 



pieces comprising the hyoid are usually lower border between the ends of the greater cornua, spread- 



(always?) distinct: 1 have never seen ing obliquely upward and fi^rward into a thin lamina with 



them anchylosed. The thyro-hyals arc slightly thickened sides, supporting the lesser cornua, and 



elongate, much compressed in an ob- having its sharp, free anterior border somewhat produced by 



lique plane, enlarged at either e.xtrem- cartilage. There is no uro- or glosso-hyal; no lytta could be 



ity, slightly curved in the continuity. found in the substance of the tongue. The cerato-hyals do 



The cerato-hyab are small, flat, triangular nodules remaining not appear to have ever formed two pieces. 



cartilaginous for a considerable time. They are mostly sup- 



