PTERASPIDOMORPH1 195 



Order 1. PTEEASPIDOMOEPHI (Heterostraci). 



It is chiefly to the works of Lankester [276J and Traquair that 

 we are indebted for our knowledge of the strange fossils included 

 in this Order. The anterior region of these fish, including the 

 head and trunk, is broad and dorso-ventrally depressed. Behind, 

 they usually narrow rapidly to the distinctly heterocercal tail, with 

 a ventral caudal fin-lobe. A single median dorsal fin is sometimes 

 (Coelolepidae), perhaps always, present. A longitudinal fold or 

 ridge on each side may possibly represent the paired fins (Traquair 

 [465-9]). Small lateral orbits are found near the anterior margin. 

 They are situated wide apart, unlike those of the Cephalaspido- 

 morphi. The mouth was probably in the form of a transverse 

 ventral slit. There is no trace of teeth or jaws. A cloacal 

 aperture has been found in Drepanaspis (Fig. 169). Thelodus 

 and Cyathaspis show signs of some six or seven branchial pouches 

 (Fig. 172). In Pteraspis, also, there is a pair of small lateral 

 apertures near the edge of the dorsal shield, which may represent 

 spiracles (Fig. 170). The exoskeleton varied from a covering of 

 scattered denticles in the Coelolepidae to plates and scales of complex 

 structure in the other families. But it is characteristic of the 

 whole group that true bone-corpuscles are never present (Huxley 

 [224]). 



The origin of the dermal plates, according to Traquair [466], is 

 as follows : In Lanarkia the whole body is covered with spine-like 

 denticles ; cones of dentine without basal plate, and with tubules 

 radiating from a central pulp-cavity, which is widely open below 

 (Fig. 167). The shagreen of Thelodus consists also of separate 

 denticles; but they are broader and flatter, and the opening of 

 the pulp-cavity is generally narrowed to a small central -aperture, 

 although no distinct basal plate is developed. These placoid scales 

 of typical dentine, with perhaps a superficial enamel, are set nearer 

 together, and may fit closely together by their crenulated edges 

 (Fig. 166). Now in the Psammosteidae the plates and scales have 

 a superficial covering of exactly similar closely fitting denticles with 

 crenulated edges, which, however, are fixed to an underlying layer 

 of spongy bone-like substance (Fig. 166, D, H). The inner surface 

 of these plates may be strengthened by a laminated layer of similar 

 substance. Thus the primitively isolated denticles appear to have 

 become connected together by a secondary development of ' bone ' 

 in the deeper layers of the connective tissue. The compound plates, 

 then, acquire a structure bearing a striking resemblance to that of 

 the cosmine-covered scales of the Osteolepidoti (p. 217). The 

 denticles, already much elongated in Psa?nmosteus, become converted 

 into smooth ridges of dentine, forming a nearly even layer over 



