430 THE ANIMAL KINGDOM. 



others descend to the sea, e.g. Anguilla, the Eel. Instances are known of 

 truly marine fish living in fresh water, e.g. a Shark in Lake Nicaragua ; 

 a Goby, Blenny, Atherina in the lakes of N. Italy. Marine fish are either 

 littoral, pelagic, or abyssal. The latter are chiefly represented by Ana- 

 canthini, a few Acanthopteri, and certain families of Physostomi. Eels 

 descend to the greatest depths. Many Cyprinoidei and Muraenoidei of the 

 temperate zones become quiescent in cold weather; and some, e.g. Carp, 

 may be frozen without loss of life. Many tropical fish, e.g. some Siluroidei, 

 Protopterus, burrow in mud during the hot season, when the rivers and 

 pools dry up, and pass into a state of quiescence, which may last appa- 

 rently for several years. 



The first remains of fish occur in Upper Silurian rocks, as spines, 

 scales, cephalic shields, part of a jaw-bone, and coprolites, probably be- 

 longing for the most part to Elasmobranchii, some perhaps to Ganoidei. 

 These two orders appear in numbers in Devonian strata, and some of the 

 Ganoidei closely approach the Dipnoi, if they are not really Dipnoans. 

 Ceratodus occurs in Permian strata, existing genera of Sharks in Cretaceous. 

 Teleostei are found in the last-named rocks, and exceed the Ganoidei in 

 numbers, but it is possible that the order is represented in the Lias. 

 Teleostei replace Ganoidei almost completely in Tertiary times. 



The Class Pisces may be subdivided into the following orders : 



I. Elasmobranchii (= Plagiostomi). Skeleton, for the most part cartilaginous; 

 no investing bones ; caudal fin heterocercal ; paired fins large ; ventral fins ab- 

 dominal ; copulatory organs present in male ; mouth transverse and ventral ; 

 nostrils ventral ; five to seven external gill-clefts, and usually a spiracle ; upper jaw 

 a moveable palato-quadrate cartilage ; optic nerves forming a chiasma ; a spiral 

 valve in the intestine; a terminal cloaca and a contractile conus arteriosus with 

 several rows of valves ; ova large and few ; impregnation internal ; viviparous or 

 oviparous ; embryo with external gills. Contains two sub-orders : (i) Squalidae or 

 Selachoidei, the Sharks, which are cylindrical in shape ; have the gill-clefts placed later- 

 ally; free eyelids; and the pectoral limb not connected to skull; (2) Rajades or 

 Batoidei, the Rays, which are flattened ; have the gill-clefts ventrally placed ; and 

 the pectoral limb connected to the skull. 



A Shark Chlamydoselachus from Japanese wafers, has lately been described, 

 which has pronged teeth like the Cladodonts of the Middle Devonian period. Its 

 mouth is anterior as in other Vertebrata; its two nostrils are on the dorsal aspect of 

 the face as in Teleostei ; it has a large free opercular fold to the hyoid ; six gill-clefts 

 and an heterocercal tail scarcely bent up. It is the oldest living type of Vertebrata. 

 See Garnian, Bull. Mus. Comp. Zool. Harvard College, xii. No. i, 1885. 



II. Holocephali differ from 1 I. in having an opercular fold covering the gill- 

 clefts, which are only four, and a naked skin ; the palato-quadrate and hyoman- 

 dibular coalesced with the skull ; no cloaca but the anus, Miillerian and urinary 

 ducts opening separately. Chimaera from the northern, and Callorhynchus from 

 the southern hemisphere. 



