336 ORDERS OF FISHES. 



both from the Secondary and Tertiary Rocks. The last divi- 

 sion of the Elasmobranchii viz., that of the Holocephali is 

 poorly represented in past time by the Mesozoic and Kaino- 

 zoic IscktoduS) Elasmodus, Ganodus, and Edaphodus. 



In the following a more detailed account is given of the 

 characters of the various groups of the Elasmobranchii, with 

 the leading characters and more important fossil forms of 

 each : 



SUB-ORDER A. HOLOCEPHALI. This sub-order includes cer- 

 tain curious fishes, of which the only living forms are the 

 Chim&ridce. The notochord is persistent, but the neural 

 arches and transverse processes are cartilaginous. The jaws 

 are bony, and are covered by broad plates representing the 

 teeth. The exoskeleton consists of placoid granules. The 

 first ray of the anterior dorsal fin is in the form of a powerful 

 defensive spine, like the " ichthyodorulites " of many fossil 

 fishes. The ventral fins are abdominal, and the tail is hetero- 

 cercal. There is only a single external gill-aperture, covered 

 with a gill-cover and branchiostegal membrane ; but only a 

 small portion of the borders of the branchial laminae is free. 

 The mouth is placed at the extremity of the head. 



The Chimaeroid fishes are not known to have existed at all 

 during the Palaeozoic period ; but they are not uncommon in 

 the Secondary period and in the Tertiaries. They are exclu- 

 sively known by means of their jaws and teeth and their fin- 

 spines or " ichthyodorulites." The dental plates are united to 

 the beak-shaped jaws; and the dorsal fin-spines are always 

 movable and jointed instead of being supported on a cartilage 

 imbedded in the muscular tissue of the back (as in the Spina- 

 cidcz and Cestrationtida). Of the fossil Chimaeroids, the genera 

 Ischiodus and Ganodus are exclusively Mesozoic ; Edaphodus 

 ranges from the Cretaceous series to the Eocene Tertiary; and 

 Elasmodus is only known from the Eocene. 



SUB-ORDER B. PLAGIOSTOMI. This sub-order is of consider- 

 ably greater importance, as it includes the well-known Sharks 

 and Rays. The vertebral centra are usually more or less ossi- 

 fied, and even when quite cartilaginous, the centra are marked 

 out by distinct rings. The skull is in the form of a cartila- 

 ginous capsule, without distinct cranial bones. The mouth is 

 transverse, and is placed on the under surface of the head. 

 The exoskeleton consists of placoid granules, tubercles, or 

 spines. The branchial sacs open externally by as many dis- 

 tinct apertures as there are sacs, and there is no operculum. 

 A pair of tubes proceed from the pharynx to open on the upper 

 surface of the head by two apertures, which are termed 



