138 



SELACHII 



One may diagnose the Selachii as : Elasmobranchs with sub- 

 divided and much concentrated median fins ; a constricted noto- 



. %. 



Ventral view of the pelvic girdle and fins of //<>/"./, us ,-;,iei-Kit.t, (Jin. Tlie .skeleton is com- 

 pletely exposed on the left side. 6, basipterygium ; tir, web of (in supported by ceratotrichia ; 

 fi, pelvic cartilage ; pr.r, series of preaxial radials. (From Quart. Jonrn. Micr. >SVi.) 



chord, an invaded notochordal sheath, and centra, if present, of 

 chordal origin ; dorsal ribs ; the two halves of the pelvic girdle 



FIG. 97. 



Diagram of the lateral-line canal of a Selachian seen in a section vertical to the surface. 

 ep, epidermis; l.c, longitudinal canal; I.n, lateral-line nerve; <>, opening of branch canal on 

 surface ; s.o, sense-organ. 



fused together ; a wall between the cavity of the auditory capsule 

 and of the brain-case, a large hyomandibular cartilage ; epibranchial 

 arteries corresponding to the branchial slits ; gill-slits opening 

 separately to the exterior ; and a rectal gland. 



The Selachii may be divided into two groups. In the first, the 



