or THE LAEVAL POLYPTERUS. 325 



In a section passing between the myomeres, and therefore through the bases of these 

 cartilages, somewhat further back, it may be seen that the section cuts small nodules of 

 cartilage at the extremity of the septa between the dorso-lateral and ventro-lateral 

 muscles (PI. XXXIV. iig. 2, o.l.r.c). These outer nodules lying near the nerve of the 

 lateral line are not, however, connected with the cartilage of the central end of the 

 septum. That is to say, the cartilage which forms the basis of the outer portion of 

 the lateral ribs is developed independently of the cartilaginous basis of the transverse 

 process. 



In the middle region of the vertebral column both the lateral and ventral cartilages 

 are small, while the dorsal cartilages of the two sides are seen to be placed in a slightly 

 alternating position, so that a transverse section passing through the centre of a dorsal 

 cartilage on the right side will pass through the posterior fringe of the dorsal cartilage 

 of the left side (PI. XXXIII. fig. 5, n.pr.). 



In the caudal region the lateral series of cartilages are not found, while the ventral 

 cartilages, though retaining their position, become the greatly enlarged haemal arches. 

 The first pair of enlarged ventral cartilages, however, do not unite ventrally, but each 

 half bears at its extremity one of the last two interspinous cartilages of the anal fin ^ 

 (PI. XXXIII. fig. 6, Lv-i^r., r.v.pr.). The two preceding interspinous cartilages of the 

 anal fin are borne by similar rods of cartilage, which, however, do not unite with the 

 ventral cartilage. 



The most anterior cartilage of the anal fin is somewhat expanded antero-posteriorly, 

 but is not supported by the haemal cartilage. 



Behind the anal fin the right and left ventral cartilages fuse together below the 

 haemal canal to form rod-shaped median cartilages, which expand distally and 

 support directly the rays of the ventral fin of the diphycercal tail of Polypterus 

 (PI. XXXIII. fig. 6, h.c). 



In this region the right and left halves of the neural arches fuse together to form the 

 dorsal spines (PI. XXXIII. fig. 6, d.sp.), while wedged in between these are found the 

 interspinous processes, which, expanding distally to trumpet-shaped masses of cartilage, 

 support the rays of the dorsal fin of the tail. There is no correspondence between 

 the dorsal spines of the neural arches and these interspinous processes, for there 

 may be one, two, or three of the latter intervening between two of the former. More 

 anteriorly these interspinous processes become much reduced and do not penetrate 

 between the dorsal spines. As the extremity of the tail is reached the neural and 



' I have satisfied myself that my description of the way in -which the anal fin is supported in this specimen 

 is correct, but I am not quite sure that this will be found to be the normal arrangement. It seems, at any 

 rate to me, to be an additional proof that the haemal arches are formed from the bending downwards and 

 uniting of originally separate ventral ribs, which have secondarily assumed tlie function of supporting the 

 anal fin. 



3a2 



