84 



COMPARATIVE MORPHOLOGY OP VERTEBRATES 



for the naso-hypophysial canal and a bar (subocular) joins the trabecula of 

 either side and in front is continued in a cornual cartilage. The Ungual car- 

 tilage is enormous (is it the lower jaw as has been suggested?), is divided into 

 three segments and bears a dental plate with teeth at its tip. There are cartilage 

 axes to the tentacles around the mouth. 



The branchial skeleton of the lampreys consists of a gill basket of continuous 

 cartilage with fenestras for the gills and above and below them as well. It can- 

 not be homologized with the branchial skeleton of other vertebrates as it lies 

 immediately beneath the skin and is lateral to gill pouches and aortic arches. 

 It is more easily compared to the extrabranchials (p. 73) of the elasmobranchs. 

 The branchial apparatus of the myxinoids is reduced, consisting of two true gill 

 arches, in front of which is another arch, usually interpreted as a hyoid. 



ELASMOBRANCHS have a nearly typical chondrocranium (fig. 84) which 

 is never divided into separate elements and is never ossified. The floor is coni- 

 plete, the hypophysis resting in a sella turcica. Above there is an anterior 



Fig. 84. — Skull of Sgualina, after Gegenbaur. h, hyoid; hm, hyomandibular; hr, 

 hyomandibular rays; l^. P, labial cartilages; m, Meckel's cartilage; pq, pterygoquad- 

 rate; r, rostrum. 



fontanelle, closed by membrane, and a posterior fontanelle may occur. The 

 occipital region typically articulates with the vertebral column by a pair of promi- 

 nences, the occipital condyles, but in most species this joint is not functional, 

 the skull being immovably united to the backbone. In front the snout is sup- 

 ported by rostral cartilages, usually three in number, but these are frequently 

 fused to a single mass. 



The pterygoquadrate and the Meckelian cartilages bear teeth and form the 

 functional jaws. Most species are hyostylic (p. 79), the pterygoquadrate being 

 supported in front of the orbit by a ethmopalatine ligament on either side; behind 

 by ligament and by the hyomandibular. In the Cestracionts the pterygoquad- 

 rate articulates with the cranium in front (fig. 78), but the hyomandibular is 

 still suspensorial. The Notidanids are amphistylic, the hyomandibular being 

 connected with the rest of the hyoid and not acting as a suspensor of the jaws, 

 but the pterygoquadrate bears a strong process which articulates with the postor- 

 bital process of the cranium. A fourth condition is found in the holocephalans 

 where the pterygoquadrate, free in the young, becomes autostylic by fusion with 

 the cranium. 



