PLEURACANTHODIAN SHARKS. 49 



absorbing nourishment from the numerous filaments or ' tropho- 

 nemata ' which project inward from the oviducal wall (see Trygon 

 bleekeri, 133, and Pteroplatea micrura, 136 a and b) . A some- 

 what similar provision for nourishing the young occurs in the 

 Piked Dog-fish and the Smooth Hound, in which the trophonemata 

 are represented by semicircular lappets of the lining of the uterus, 

 each with a blood-vessel passing round the free edge. 



PLEURACANTHODES (Pleuracanthodian Sharks). 



The order Pleuracanthodes includes Palaeozoic Sharks of 

 primitive type (fig. 26), the remains of which occur in rocks 

 ranging from the Lower Carboniferous to the Lower Permian. 

 The cartilages are permeated with minute granular calcifications, 

 and the cranium sometimes possesses a curious symmetrical 

 Assuring, although there are no true membrane bones. Slight 

 calcifications sometimes occur in the sheath of the notochord. 

 The paired fins have a long segmented axis of cartilage, fringed 

 one or on both sides with cartilaginous fin-ravs, to the 



on 



rays 



acanthus. 



extremities of which the dermal fin-rays are attached in bunches 

 (see sketch 141). The median fins are extensive. There is no 

 shagreen, but small scattered tubercles occur in the skin, and 

 there is a median spine projecting from the back of the head. 



The restored sketch (natural size) of Pleur -acanthus decheni, 140, Pleur- 

 from the Permian Beds of Bohemia, shows the principal characters 

 of these Sharks; the mouth is nearly terminal (fig. 26), the tail 

 tapers evenly and symmetrically, the upper part of the caudal fin 

 is separated by a short break from the dorsal fin, which extends 

 forward nearly to the head, and there are two small anal fins. 

 Remains of other species of Pleur acanthus occur in the Coal 

 Measures of France, England, and America. The teeth (see 

 sketch 142) are tricuspid, but the middle denticle being compa- 

 ratively minute the name Diplodus is commonly applied to isolated 

 eeth of the genus. 



HOLOCEPHALI (Chinueroid Fishes). 



The Chimseroid fishes are fishes of grotesque appearance, Wall- 

 occurring mostly in deep water, and related most nearly to the case ~ K 

 Elasmobranch fishes (Sharks and Rays), although their dentition 



