SEXUAL CHARACTERS — ELASMOBRANCH FISHES 225 



reveals (fig. 3) that both cavities are Hned with stratified 

 epitheUum. 



The apex of the cavity cannot be attained without cutting through 

 the cartilages a - a', b - b' (fig. 2) ; in this it differs from a siphon, 

 but resembles the cavity of Chimaera. On either side of the 

 cavity is a deep sulcus leading down to the aperture of the cavity 

 at the apopyle. 



SPINAX (ETMOPTERUS) NIGER 



The black dogfish 



This smallest of the elasmobranchs affects deep water. I 

 have examined specimens from 200 fathoms, from Christiansund ; 

 from 200 fathoms, from the coast of Portugal, and the specimen 

 from which the figures are drawn was taken in 1904 at Faro, 

 Algarve, Portugal, from 365 fathoms, and is 26 cm. long. 



This memoir deals largely with types like Acanthias which 

 depend upon movable spines rather than upon a rhipidion for 

 fixative purposes during impregnation. These, in Spinax, are 

 four in number, set within a senticetum which can be erected, 

 in which case they take up the positions indicated in figure 4. 

 One, number 3, points forward, while number 4 differs from the 

 rest in being fiat and blade-like, with a keen edge. The apex (a) 

 of the senticetum is a tapering flap which projects over the 

 spines, which, in a position of rest, are, according to Jungersen 

 (b. Memoir I, p. 265), hidden by a pair of cartilaginous plates 

 covered by skin. 



The claspers are stout, denticled all over, and adnate with the 

 pelvic fins. There is no rhipidion. 



A series of embryos and young forms, taken at Bergen in 1901, 

 and specially drawn for me by Michael G. L. Perkins at the 

 University Museum, Cambridge, England, exhibits very clearly 

 the development of the claspers and their state of coalescence 

 with the pelvic fins (fig. 5) . 



The siphon is peculiarly stout, and may be mistaken at first 

 sight for a gland. Sectioning, however, reveals that its solidarity 

 is due to the extraordinary thickness of its muscular walls, 

 which are here developed to an unusual extent (fig. 6). The 



