﻿36 CHIM3GR0IDEI. 



The basal cartilage (b) of the fin is evidently almost as 

 long as the upper part of the supposed clavicle, with a very 

 broad, triangular, distal extremity, meeting the obliquely 

 truncated, attached end of the pectoral spine (s), and ter- 

 minating in a very slender, rounded, proximal half. The 

 fin-membranes are shown both in connection with this 

 and all the other fin-spines ; and there are four pairs of 

 broad, intermediate ventral spines, increasing in size 

 posteriorly. Purchased, 1880. 



Subclass II. HOLOCEPHALI. 



Skeleton cartilaginous, membrane-bones absent. Mandibular sus- 

 pensorium and upper jaw fused with the cranium. Exoskeleton, 

 when present, structurally identical with the teeth. In the living 

 forms — optic nerves not decussating, bulbus arteriosus of the heart 

 with three series of valves, intestine with a spiral valve, and ovaries 

 with few large ova. 



Order CHIM.EROIDEL 



Notochord persistent or partially constricted, the calcifications in 

 the sheath, when present, consisting of slender rings more numerous 

 than the neural and haemal arches. Pectoral fins shortened, without 

 segmented axis ; pelvic fins produced into a pair of claspers in the 

 male. In the living forms — a fold of skin covering the gill-clefts, 

 and leaving a single external opening to the gill-cavity. 



In all the known families of Chimseroids, the dentition consists 

 of few large plates of vascular dentine, of which certain areas 

 (" tritors ") are specially hardened by the deposition of calcareous 

 salts within and around groups of medullary canals, which rise at 

 right angles to the functional surface. In most cases there is a 

 single pair of such plates in the lower jaw, meeting at the sym- 

 physis, while two pairs are arranged to oppose these above. As a 

 whole, the dentition thus closely resembles that of the typical 

 Dipnoi (as has often been pointed out) ; and the upper teeth may 

 be provisionally named palatine and vomerine until further dis- 

 coveries shall have revealed their precise homologies. The struc- 

 tures are sometimes described as "jaws," and regarded as dentaries, 

 maxilla?, and premaxillae, but the presence of a permanent pulp 



