PLEURACANTHODII 



185 



the tail. Two dorsal fins are present, but apparently no anal. 

 There are paired pectoral and pelvic fins of remarkable structure. 

 No rostral process projects beyond the mouth ; the orbits are 

 surrounded by a double ring of dermal plates, probably formed of 

 fused denticles. Small denticles are distributed over the body; 

 their histological structure has unfortunately not yet been described. 

 The lateral line runs between two rows of denticles. Broad-based 

 teeth, with a large median and smaller lateral cusps, are closely 



FIG. 152. 



Chidoselache Fyleri, Newb. ; Upper Devonian, Ohio. A, right-side view ; J3, ventral view ; 

 C, front view; restored. (After Dean, from A. S. Woodward, 1'roc. Ccol. Azsoc.) 



set in rows transverse to the jaws (Fig. 153). The skull resembles 

 .that of the Notidanidae ; also the jaws, which are amphistylic, 

 with a broad palato-quadrate cartilage and long backwardly directed 

 hyomandibular. The branchial arches were five, or possibly six or 

 seven, in number. The 

 notochord was probably 

 unconstricted ; no centra, 

 but neural and haemal 

 arches without inter- 

 calaries, have been found. 

 The endoskeletal supports 

 of the median and paired 



fine oro iini^i'^tnrl ovfonrl Teeth of Cladoxlache, from different regions of the jaws. 



tins are unjointed, extend (After Dean> Fi .^cs Living and Fossil.) 



io near the edge of the 



fin-web, and the ceratotrichia are but little developed. Possibly 

 this extension is primitive; but it must be noted that it occurs 

 also in the Rajiformes and certain other Selachians. The dorsal 

 lobe of the caudal fin is borne by segmental radials, and the ventral 

 by numerous slender cartilages crowded in its lower half (Fig. 154). 



FIG. 153. 



