82 LOWER VERTEBRATES. 



Best known is the smooth hound, or dog-shark, Mustelus canis, figured on page 

 77, together with Squalus acanthias. It is a small species, the smallest of the Ameri- 

 can sharks, rarely reaching a length of four feet. In its range it includes the Atlantic 

 coasts of both continents, on our shores being more common south than north of Cape 

 Cod. It also occurs on the coast of California, where it was originally described as a 

 distinct species under the name M. californicus. It is harmless, and feeds on the 

 crabs and shells which abound on the bottom in favored localities. From its resem- 

 blance to the dog-fish, it is often confused with that species, but is readily separated 

 by the lack of spines in front of the dorsal fins. 



In the genus Mustelus all the teeth are obtuse, and there is no internal connection 

 between the mother and the embryos. The young are born ali^e, sometimes a dozen 

 at a birth. 



In the genus Rkinotriacis of which R. henlei occurs on the Pacific, and J2. loevis 

 on the Atlantic coast (the latter being more properly an European species), the teeth 

 are sharper, some having basal cusps, while a placenta is developed to assist in nour- 

 ishing the growing embryo. In Europe, and especially in Italy, the species both of 

 lltistelits and of Mhinotriacis, are eaten by the poorer people, although the flesh is 

 not very palatable. Both share the name of hound, and smooth dog-fish. This appli- 

 cation of the names dog, hound, porbeagle, etc., to different species of sharks, is 

 doubtless due to their following theii- prey in packs. 



The remaining genera to be mentioned of the family have the teeth acute, and 

 with sharj) edges. The genus Galeorhinns, or Galena, frequents the warmer waters. 

 G. galeus is called tope in England ; it extends its range to Tasmania and Californin, 

 but has not yet been found on the east coast of the United States. In California it 

 is common, and has received the common name of oil-shark. It proves itself a nui- 

 sance to the fishermen, since it not only takes the bait itself, but drives away the 

 other fishes. The topes are small sharks. The species of Galeocerdo are larger. G. 

 ' tigrinus occurs on both Atlantic and Pacific coasts, and derives its common name 

 tiger-shark from its variegated coloration. 



The genus Carcharinics embraces the blue-sharks, the sharks of story. The species 

 are numerous, especially in the warmer seas, and if we take the genus in its wider 

 sense, it embraces about forty nominal species. The genus may be separated from 

 the rest of the family by the serrated well-developed teeth, and the absence of a 

 spiracle. The great blue-shark, C. glaucits, is the best known of the genus ; it ranges 

 through all the seas, and occasionally occurs on the coast of the eastern states, as well 

 as on the Californian shore. In size it commonly reaches a length of about fifteen 

 feet, but individuals twenty-five feet or more in length are sometimes seen. It is blue 

 above, and white beneath. The v.-hite-shark, C. vulgaris, is even larger, a specirnen 

 thirty-seven feet in length, having been known. Its color is grayish-brown above, and 

 white below. It is an inhabitant of the tropical seas, and is but rarely seen in tem- 

 perate zones. More common on our coasts is the dusky-shark, C. obscurus, which 

 rarely exceeds ten feet in length, and, like the great blue-shark, is blue above. An 

 important difference exists in the position of the dorsal fins in the two. In C. glaucus, 

 the dorsal is midway between the pectorals and ventrals, and in the dusky-shtirk, it is 

 placed close behind the root of the pectorals. These facts, as well as the existence of 

 a placenta, have led Dr. Gill to separate it and a few other species, as a distinct genus, 

 Mulamia. 



The species of Carcharinus share with the species of Carcharodon the name 



