SELACHOIDEI. 319 



of the body. Spiracles small, behind the eyes. Teeth small, 

 numerous, similar in both jaws, obtuse, or with very indistinct 

 cusps, arranged like pavement. 



The " Hounds " are small Sharks, abundant on the coasts 

 of all the temperate and tropical seas ; two of the five species 

 known occur on the coasts of Europe, viz. M. Imvis and 

 M. vulgaris. Closely allied as these two species are, they yet 

 show a most singular difference, viz. that a placenta is de- 

 veloped in the uterus for the attachment of the embryo in 

 M. Imvis (the FaXeo? Xeto? of Aristotle, to whom this fact 

 was already known) ; whilst the embryons of M. vulgaris are 

 developed without such placenta (see J. Muller, " Abhandl. 

 Ak. Wiss." Berl. 1840). The Hounds are bottom fish, which 

 feed principally on shells, crustaceans, and decomposing 

 animal substances. 



Several other genera belong to the family Carchariidce, 

 but it will be sufficient to mention their names : — Hemi- 

 galeus, Loxodon, Thalassorhinus, Trioenodon, Leptocarcharias, 

 and Triads. 



Second Family — Lamnid^e. 



Eye without nictitating memhrane. Anal finpresent. Two 

 dorsal fins; the first opposite to the space between pectoral and 

 ventral fins, without spine in front. Nostrils not covfiuent 

 with the mouth which is inferior. Spiracles absent or minute. 



All the fishes of this family attain to a very large size, and 

 are pelagic. But little is known of their reproduction. The 

 first appearance of this family is indicated by Carcharopsis, 

 a genus from carboniferous formations, the teeth of which 

 diifer from those of Carcharodon only by having a broad fold 

 at the base. In the chalk and tertiary formations almost 

 all the existing genera are represented ; and, besides, Oxytes, 

 Sphenodus, Gomphodus, and Ancistrodon, which are known 



