XIII 



PHYLUM CHORDATA 



253 



apparently cartilaginous ; the mandible was probably autostylic. 

 There were composite cutting dental plates. The pectoral fins are 

 unknown ; the rays of the small pelvic ( VR) were supported on a 

 flattened plate (VB). 



With some special features of their own the Dipnoi combine 

 characteristics in which they resemble now one, now another, of 

 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 and even the higher Vertebrates. The brain and the 

 heart are quite peculiar : the former in its undivided, or almost 

 undivided, mid-brain ; the latter in its imperfectly divided auricle, 

 and spirally-twisted conus. The pallium of the cerebral hemispheres 



N ~, OR 



FIG. 026. Cocccsteus decipiens. Side view, restored. A, articulation of head with 



trunk DB, cartilaginous basals of dorsal fin ; DP,, cartilaginous radials of dorsal fin ; 



77, hsemal arches and spines ; MC. mucous canals ; JV, neural arches and spines, and 



. position of notochord ; U. median unpaired plate (?) of hinder ventral region ; VB, liasaN 



of pelvic fiu ; I'/?, radials of pelvic fin. (From Dean, after Smith Woodward.) 



in the Dipneumona with its layers of nerve-cells has no parallel 

 among the lower Vertebrates. In the Limbs the Dipnoi are only 

 closely approached by certain extinct Elasmobranchs (p. 163). In 

 the presence of a cloaca and a spiral valve they also approach that 

 sub-class, as well as in the contractile conus the last two 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 higher Vertebrates. 



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 

 endoskeleton, including jaws. It may therefore be assumed that there was 

 a persistent 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. 



