54 PISCES. 



remarkable for the large size of the iliac and prepubic pro- 

 cesses of its pelvic cartilage (fig. 43). The Torpedinidae, so 

 far as known, are typically Tertiary, but skeletons apparently 

 referable to Platyrhina and Narcine occur in so early a for- 

 mation as the Eocene of Monte Bolca, near Verona. 



Sub-Class 2. Holocephali. 



Dental plates essentially similar to those of the existing 

 Chimaeroid fishes are met with in rocks as early as the Middle 

 Devonian ; but there is still no evidence of any member of the 

 Holocephali which cannot be included in the surviving order of 

 Chimaeroidei. Some of the early forms were certainly armed 

 with dermal plates ; but Palaeontology as yet lends no support 

 to the theory that the Chimseridae are degenerate descendants 

 of fishes once possessed of membrane bones. The earliest 

 known complete skeletons are unfortunately only Liassic. 



ORDER 1. CHIIVLEROIDEI. 



In all known Chimaeroids, whether recent or extinct, the 

 notochord is persistent and at most only partially constricted, 

 the calcifications in the sheath, when present, consisting of 

 slender rings more numerous than the neural and haemal 

 arches. The pectoral fins are abbreviate, without segmented 

 axis ; and the pelvic fins in the male are produced into a pair 

 of claspers. In the extinct forms there is no trace of any 

 dermal plate developed in the opercular flap. The only clear 

 evidence of evolution hitherto observed concerns the develop- 

 ment of the peculiar dental plates. In each of the four known 

 families the dentition consists of a few large plates of vascular 

 dentine of which certain areas ("tritors") are specially hardened 

 by the deposition of salts within and around groups of medullary 

 canals, which rise at right-angles to the functional surface. In 

 most cases there is a single pair of such plates in the lower 

 jaw, meeting at the symphysis, while two pairs (the so-called 

 vomerine and palatine plates) are arranged to oppose these 

 above. A permanent pulp remains under each plate, and 

 growth thus takes place continually within as the oral surface 



