62 THE VERTEBRATE SKELETON. 



column, is however often calcined, this being especially well 

 seen in the anterior part of the vertebral column of Rays 

 (Raiidae). In living forms cartilaginous biconcave vertebrae 

 are always well developed, but in some extinct forms the 

 notochord persists unconstricted. Neural and haemal arches 

 are however always developed ; they sometimes remain 

 separate, sometimes fuse with the centra. Ribs are often 

 wanting and when present are often not separated off from the 

 vertebrae. The cranium is a simple cartilaginous box whose 

 most prominent parts are the capsules which enclose the 

 sense organs. The skull is sometimes immovably fixed to 

 the vertebral column, sometimes articulates with it by means 

 of two condyles. There is no operculum and no representative 

 of the maxilla or premaxilla. The teeth are very variable. 

 Large pectoral and pelvic fins always occur. 



The Elasmobranchii may be divided into four suborders : 



(1) Ichthyotomi. 



(2) Pleuropterygii. 



(3) Selachii. 



(4) Acanthodii. 



Suborder (1). ICHTHYOTOMI \ 



The members of this suborder range from the Devonian to 

 the Permian and so have long been extinct. 



The endoskeletal cartilage has granular calcifications evenly 

 distributed throughout it. The notochord is unconstricted, but 

 the neural and haemal arches are well-developed, and the 

 neural spines are long and slender. There is a continuous 

 dorsal fin with separate basalia and radialia. The tail is di- 

 phycercal, and the pectoral fins are typical archipterygia -. 

 The pelvic fins of the male are prolonged to form claspers. 



1 For this arid other groups of extinct fish see A. Smith Woodward, 

 Catalogue of Fossil Fish in the British Museum, Parts i. in. London, 

 188995. 



2 See p. 127. 



