xrir PHYLUM CHOIiDATA 2(;] 



Elasmobranchs (p. 168). In the presence of a cloaca and a 

 spiral valve they also approach that sub-class, as well as in the 

 contractile conns — the two last features being also shared with the 

 Ganoid Teleostomi. The operculum with its supporting bones 

 connects them with the Teleostomi. The Amphibian features will 

 be referred to at a later stao^e. On the whole, though in some 

 respects more primitive than the members of the other sub-classes 

 of Pisces, the Dipnoi tend to establish a connection between that 

 class and the Amphibia. 



APPENDIX TO PISCES. 



THE OSTRACODERMI 



The Ostracodernii are a group of Paleozoic Fishes of uncertain affinity, 

 characterised In' the extraordinary development of the exoskeleton of the liead 

 and trunk, and the absence, in all the fossil remains hitherto found, of endo- 

 skeleton, including jaws. It may therefore be assumed that there was a per- 

 sistent notochord, and that the rest of the skeleton was unossified. It is 

 uncertain whether the group should be considered the equivalent of a Class 

 or of a Sub-class : it is divisible into three onlers, which are best considei-ed 

 separately. 



Order 1. — Heterostraci. 



This order includes four families, tlie Pteraspidoi the Coelolepidce, the 

 Drepana-spidce and the Psammosteidcp. Of the first Pierasjns (Fig. 916) may be 

 taken as an example. The body is elongated, and divided into an anterior 

 region, representing the head and fore-part of tlie trunk, and covered by strong 

 calcified plates or scutes, and a posterior or caudal region covered by rhomljoidal 

 scales. In the anterior region there are seven scutes above, constituting the 

 dorsal shield, while below there is a single ventral shield. The dorsal shield is 



Fig. 910.— Pteraspis rostrata (Uevoniau). (From the Brit. Mus. Cat. of Fossil Fishes.) 



produced into a rostrum, and is hollowed by a pair of lateral orbits, Ijetween 

 which is a pit, on the inner surface of the shield, probably marking the position 

 of the pineal l)o<ly. The scutes contain no lacun;e or canalicidi, and liave not, 

 Therefore, the structure of bone : they are lined by a nacreous layer, and are 

 covered externally M'itli a lajer of vaso-dentine. The tail appears to have been 

 heterocercal. A pair of longitudinal ridges ma}' represent paired fins. 



The Ca-lole2ndit (Fig. 917) have the head and anterior trunk z-egion flattened and 

 expanded, with postero-lateral lobes which may I'epresent paired fins. There is 

 a heterocercal tail-fin. Mouth, orbits, and branchial apertures have not 

 been detected. The exoskeleton takes the form of numerous uniform, hollow, 



VOL. II R 



