5 24 



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 cliaracteristic 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 Ijranchial 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.^ — 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. 

 ISTothing 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 llielodus 

 2Mgei may be indications of a bran- 

 chial apparatus. The exoskeleton 

 consists of a uniform covering of 

 hollow pointed spines, devoid of a 

 basal plate and open below {Lcm- 

 eaoh side a much enlarged dermal arkio) ; or of minute shagTeen-hke 



denticle IS shown. (FromTraquair.) , i -< , rm i -, ^ rx,, , 



^ ' inhevdes {Thdodus). The tubercles 

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

 known genera, Thelodus is a characteristic Upper Silurian genus 



' Traquair, Trans. Roy. Soc. Edinl. xxxix. 1899, pp. 595 and 828. 



Fig. 313. — Restored outline of Lan- 

 a.rkia spinosa, in the position in 

 which it occurs as a fossil, the head 

 being flattened and the tail twisted 

 round so as to appear in profile. On 



