974 CLASS PISCES. 



rays of the dorsal and caudal fins are divided and furnished with 

 spines. In the English Liassic Holophagus the spines on the 

 scales are more numerous, while in Libys, from the Kimeridgian 

 of Bavaria, they are less numerous, the division of the fin-rays 

 extends more deeply than in Undina, and the dorsal and caudal 

 scales have a row of tubercles. Coccoderma, from the Kimeridgian 

 of Europe, is allied to the last. Finally, Macropoma (fig. 910), 

 from the Chalk, comprises several large species readily characterised 

 by the notochord not extending to the extremity of the tail. 



Family Polypterid^e. — The last family of the suborder is known 

 only by the existing African genera Polypterus (fig. 906, a) and 

 Calamoichthys, each of which is represented by a single species. 

 The vertebral column is ossified ; the dorsal fin broken up into a 

 number of small finlets ; the pectorals are obtusely lobate ; the 

 caudal fin is diphycercal, with a very short body-axis ; and the scales 

 are rhomboidal. 



Suborder 5. Acipenseroidea. — According to the views of Dr 

 Traquair, the Acipenseroids, represented typically by the Sturgeons 

 and their allies, and forming the Chondrostei of many authors, are 

 also taken to include the Heterocerci, as represented by the extinct 

 PalceoniscidcE. The following are some of the leading features of 

 the group as thus defined. The paired fins are non-lobate ; the 

 pectoral girdle, which in the typical forms retains its primitive 

 endoskeletal cartilages, develops dermal bones, among which the 

 infraclavicular (fig. 913) is characteristic; the dermal rays of the 

 dorsal and anal fins are more numerous than their supporting carti- 

 lages, or interspinals, of the endoskeleton ; while in the paired fins 

 these dermal rays have to a great extent replaced the original carti- 

 lages. In the skull the cartilaginous cranium persists in the typical 

 forms, but in all cases it is overlain by a series of dermal bones ; the 

 preopercular, when present, tends to extend on to the cheek ; bran- 

 chiostegal rays are generally present ; but there are never large 

 jugular plates. The notochord is persistent, but there are either 

 cartilaginous or bony neural and haemal arches ; the tail is hetero- 

 cercal, and the skin may be either naked, or dotted over with bony 

 scutes, or covered with rhomboidal scales. 



Mr Smith Woodward remarks that the typical forms of this sub- 

 order constitute a link connecting the cartilaginous Ganoids with 

 these fishes in which the bones are fully developed, and that their 

 paired fins are more specialised than the median ones, which have 

 not yet attained a numerical equality between the fin-rays and their 

 supporting interspinals. 



Family Acipenserid^e. — The well-known Sturgeons (fig. 911), 

 which form the typical family, are large fresh-water fishes, charac- 

 terised by their elongated body, produced snout, toothless jaws of 



