524 



FISHES 



structure in the different families, ranges from a uniform cover- 

 ing of dermal denticles to a condition in which the denticles 

 fuse to form anteriorly a highly characteristic tessellated or 

 continuous dorsal shield, while posteriorly they become replaced 

 by a nearly typical rhombic squamation. The tail is hetero- 

 cercal. Paired fins of the ordinary piscine type are absent. In 

 some Ostracodermi it seems probable that the gill-clefts opened 

 into a common branchial chamber on each side, with a single 

 external aperture, but in others they may have been ventral. 

 The endoskeleton, jaws, dentition, and the nostrils are unknown. 



Order I. Heterostraci. 



The exoskeletal structures consist of dentine, or of a tissue 

 resembling it, never of true bone. The orbits are marginal or 



lateral in position. With the ex- 

 ception of a caudal fin there are 

 no median fins. 



Fam. 1. Coelolepidae. 1 Head 

 and anterior portion of the body 

 flattened and expanded, with promi- 

 nent lappet - like postero - lateral 

 lobes, which may represent continu- 

 ous lateral fin - folds or a very 

 primitive type of pectoral fin. 

 Nothing is known of the mouth, 

 but it must have been ventral, nor 

 of the position of the orbits. 

 Branchial apertures unknown, but 

 transverse markings on each side 

 of the anterior part of Thelodus 

 payei may be indications of a bran- 

 chial apparatus. The exoskeleton 

 FIG. 31 3. -Restored outline of Lan- cons i s t s o f a uniform covering of 



arkta spinosu, in the position in . . 



which it occurs as a fossil, the head hollow pointed Spines, devoid of a 



being flattened and the tail twisted bagal kte &nd Mow ^ (m _ 



round so as to appear in profile. On * r t 



each side a much enlarged dermal arkia) ] Or of minute shagreen-like 



denticle is shown. (From Traquair.) tubercleg (Thdodus}. The tllbei'deS 



or spines consist of dentine coated by ganoin. Of the only two 

 known genera, Thelodus is a characteristic Upper Silurian genus 



1 Traquair, Trans. Roy. Soc. Edinb. xxxix. 1899, pp. 595 aud 828. 



