Oct. I, 1S74J 



JVA TURE 



44 



unaided the whole of the anteiioi- third of the brain- 

 case — roof, walls, and floor. There is yet another im- 

 portant feature in these curious bones — the cylindrical 

 cavity which tliey enclose is divided in front by a double 

 pillar of bone, to which each frontal contributes its own 

 half, and on either side of which the olfactory nerves pass 

 to the nasal sacs : in this way a remarkable resemblance, 

 both in form and position, to the frog's "girdle-bone" is 

 produced ; an analogy, indeed, which only the study of 



S.h, 



-Skull of Sn J:e (..pp=; 



cw). Tr. Oi tr: 



development can show to be as far as possible from a true 

 homology. 



Interposed between the anterior border of the ex- 

 occipital and the posterior border of the descending 

 portion of the parietal, is a stout irregular bone, which 

 anyone studying the adult skull only would certainly 

 look upon as the periotic or ossified otic capsule. As a 



P-n'L 



FiC. 2!.— Skull of Snake (under view). .Spl, iplcnial ; Cor, coronary ; Anj, 

 anguLar ; S. Ang, snpra-aogular. 



matter of fact, however, it is bo'.h more and less than this. 

 In the young state it consists of two perfectly distinct 

 ossifications, between which the fifth nerve makes its exit. 

 Now, this nerve (sec N.\Tt;RK, vol. x., p. 10) marks the line 

 of demarcation between ihe posterior boundary of the 

 parietal segment and the auditory capsule; the bone in front 

 of it is, therefore, the alisphcnoid, and that behind it the 

 prootic, the latter being further determined by the fart 

 that i' lo Igcstlie main partof the vestibule, of the anterior 



and horizontal canals, and of the rudimentary cochlea. 

 The remaining elements of the ear-capsule are, in the 

 adult, quite undistinguishable ; it is seen, however, that 

 the arch of the posterior canal, as far forward as its junction 

 with the anterior, extends into what appears to be the 

 supra-occipital, and that the ampulla of the posterior and 

 the hinder portion of tlie horizontal canals invade, in like 

 manner, the ex-occipital. The explanation of this seem-ng 

 anomaly — so common in the Sauropsida — is to be found 

 in the snake at the time of hatching, when the pro-, epi-, 

 and opisthotic elements are perfectly distinct from the 

 neighbouring bones as well as from one another : as 



Fig. 21.— Skull of Snake (side 



th jaws removed). Col, colum-lla, 

 connected by a 



growth proceeds the epiotic becomes firmly anchylosed 

 with the supra-occipital, and the opisthotic with the ex- 

 occipit.al : the prootic, at the same time,remaining separate 

 from the bones with which it is naturally related, acquires 

 an intimate connection with the alisphenoid, forming with 

 it the seeming " periotic " of the adult snake. 



At the sides of the frontal region, and forming the 

 anterior and posterior boundaries of the orbit, are two 

 representatives of the "lateral line series" so prominent 

 in osseous fish : these are the antorbital and the post- 

 orbital. The antorbitals are large triangular bones, and 

 between them lie the nasals, which together have a rhom- 

 boid form, and the inner edges of which are turned down- 

 wards, forming vertical plates similar to the inter-olfactory 

 pillars of the frontals. In front of the nasals, and form- 

 ing the termination of the snout, is the small toothless 

 prcmaxilla, an azygos bone, with short nasal, maxillary, 

 and palatine processes. The vomers are two hollow, 



24,— CKondri 



I of Embryo Snake (upper view), p.b.c f, pos 

 or basi-cranial fonUnello. 



scroll-shaped bones, bearing on their excavated upper 

 surfaces the nasal glands ; the ducts of these piss 

 through a notch in the outer border of the vomers, which 

 is converted into a foramen by means of a triangular 

 ossification, the 5cpto-m.axillary, here attaining its greatest 

 development. The duct of the nasal gland is also sup- 

 ported on the outer side by two labial cartilages (1). 



The foregoing bones are all compactly united with one 

 another ; the remaining ones, forming the powerful man- 

 ducatory apparatus of the upper and lower jaws, are 

 .Trticulatcd only by loose fibrous tissues, and are thus 



