FISHES 467 



characteristic, and widely spread genera are new. Of these may 

 be mentioned : Aiietites, /Egoceras, Harpoceras, Stephanoceras, 

 Perisphinctites, and many others, each with large numbers of spe- 

 cies. The Belemnites, which were introduced in a small way in 

 the Trias, in the Jurassic blossom out into an incredible number 

 of forms, exceeding even the Ammonites in abundance of indi- 

 viduals, if not of species. These extinct Cephalopods belonged 

 to the Dibranchiata, as do all the living forms except the pearly 

 Nautilus ; they in some measure serve to connect the extinct 

 genera having external shells with the existing naked squids and 

 cuttle-fishes, which have only rudimentary internal shells, the pen 

 or cuttle-bone. The Belemnites have a straight, conical, cham- 

 bered shell, called the phragmocone, which ends above in a broad, 

 thin plate. The phragmocone was partly external to the animal, 

 and its lower, pointed end was inserted into a dart- or club-shaped 

 body called the guard or rostrum (PL IX, Fig. 4), which is com- 

 posed of dense, fibrous, crystalline calcite. Usually only the guard 

 is preserved in the fossil state, and specimens are so common that 

 they have attracted popular interest and bear the folk-name of 

 " thunderbolts." In a few instances the animal has been preserved 

 almost entire, so that the structure is well understood. 



Vertebrata. — The Fishes have advanced much beyond those 

 of the Trias. The Sharks have attained practically their modern 

 condition,. and the broad, flattened Rays are a new type of the 

 order. The Chimceroids were much more numerous and rela- 

 tively important than they are at present, when only a few are 

 left. The Dipnoans have become very scarce and are hardly rep- 

 resented in the Northern Hemisphere, save for the persistence of 

 Ceratodus. The Crossopterygians are greatly reduced, though a 

 few exceedingly curious forms, like Undina, still linger. Of the 

 Teleostome fishes the Ganoids are still the dominant type, as they 

 had been since the Devonian. Some of these Jurassic forms are 

 evidently the forerunners of the Sturgeons, but most of them re- 

 semble the Gar-pike of our Western rivers {Lepidosteus), and are 

 covered with a heavy armour of thick, shining, rhomboidal scales. 

 Many of these Ganoids are of small or moderate size, such as 



