xin PHYLUM CHORDATA 243 



the other groups of Fishes, together with a few in which they 

 approach the next class of Vertebrates to be dealt with, viz. the 

 Amphibia. The brain and the heart are quite peculiar : the former 

 in its undivided, or almost undivided, mid-brain ; the latter in its 

 imperfectly divided sinus and auricle, and spirally twisted conus. 

 In the limbs the Dipnoi are only closely approached by certain 

 extinct Elasmobranchs (p. 155). 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 best 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 Ostracodermi are a group of Palaeozoic Fishes of uncertain affinity, 

 characterised by the 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 

 imcertain 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. HETEROSTRACI. 



This order includes a single family of three genera : Pteraspis may be taken as 

 an example (Fig. 866). The body is elongated, and divid ed into an anterior region, 



FIG. 866. Pteraspis rostrata (Devonian). (From the Brit. Mus. Cat. of Fossil Fishes.) 



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 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 lacunae or canaliculi, and have not, therefore, the 

 structure of bone : they are lined by a nacreous layer, and are covered externally 

 with a layer of vase-dentine. The tail appears to have been heterocercal, but 

 there is no trace of paired fins. 



R 2 



