xm 



PHYLUM CHORDATA 



169 



Sharks the hyoid is usually relatively massive ; in the Rays it is 

 smaller, and in most cases closely resembles the branchial arches, 

 and bears similar cartilaginous rays ; a larger or smaller median 

 element, or basihyal, is present in all cases. 



There are always five pairs of branchial arches except in Hexan- 

 chus and Chlamydoselachus, which have six, and Heptanchus, 

 in which there are seven. Their dorsal ends are free in the Sharks, 



bcis.br- 



FIG. 846. Skeleton of Sting-Ray (I'mlo/ilnts), ventral view. a. v. p. anterior vertebral 

 plate ; has. br. basi branchial plate ; br.l br.5 branchial arches. (The branchial 

 rays are not represented, "the round dots indicating their articulations with the arches.) 

 cl. skeleton of clasper ; fi. m. hyomandibular ; hij. hyoid arch ; lab. labial cartilage ; lig. 

 ligament connecting the hyomandibular with the palatoquadrate and Meckel's cartilage ; 

 mck. Meckel's cartilage ; ms. pt. mesopterygium, and mt. fit. metapterygium of pectoral tin ; 

 mt. pt'. metapterygium of pelvic fin ; nas. nasal cartilage ; pal. palatoquadrate ; pect. 

 pectoral arch ; pi. pelvic arch ; pro. pt. propterygium ; sp. spiracular cartilage. 



articulated with the anterior vertebral plate of the spinal column 

 in most Rays. Externally they bear a series of slender cartilaginous 

 branchial rays. The median ventral elements of the branchial arches 

 are usually more or less reduced, and in some cases are represented 

 by a single basi-branchial plate (Fig. 846 bas. br.). In the Rays 

 the fifth branchial arch articulates with the pectoral arch, a connec- 



