138 SELACHU 



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 tins of Heptanchus dm n us, Gm. The skeleton is com- 

 pletely exposed on the left side, h, basipterygium ; dr, web of I'm supported by ceratotrichia ; 

 i'. pelvic cartilage ; pr.r, series of preaxial radials. (From Quart. Joum. Mir,-. Sci.) 



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

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



lc 



Fig. '.'7. 



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

 ep, epidermis; /.'•, longitudinal canal; './/, lateral-line nerve; o, opening of branch can 

 surface: 5.0, sense-organ. 



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

 and of the brain-case, a large hyomandibular cartilage ; epibranchi.il 

 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 



