XIII PHYLUM CHORDATA 261 



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 conus — 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 stage. 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 Ostraoodermi are a group of Palaeozoic Fishes of uncertain affinity, 

 characterised by tlie extraordinary development of the exoskeleton of the head 

 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 orders, which are best considered 

 separately. 



Order 1. — Hetbrostraci. 



This order includes four families, the Pteraspidm the Goslolepidce, the 

 Drepanaspidce and the Psammoateidce. Of the first Pleraspis (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 the trunk, and covered by strong 

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

 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. 916. — FteraspiS rostrata (Devonian). (From the Brit. Mus. Cat. of Fossil Fishes.) 



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

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

 of the pineal body. The scutes contain no lacuna or canaliculi, ■ and have not, 

 therefore, tlie structure of bone : they are lined by a nacreous layer, and are 

 covered externally with a layer of vaso-dentine. The tail appears to have been 

 heterocercal. A pair of longitudinal ridges may represent paired fins. 



The Gcclolepidie (Fig. 917) have the head and anterior trunk region flattened and 

 expanded, with postero-lateral lobes which may represent 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 



