FISHES OF THE GENUS ACENTROPHORUS. 33 



The iiLimbei' of vertebral segments seems to agree closely with 

 the number of rows of scales — about forty in each case fi'om the 

 head to the middle of the base of the caudal fin — but the longi- 

 tudinal spacing does not correspond by any means. In the caudal 

 region ihe segments are so widely spaced as to suggest some sort 

 of foreshadowing of the alternate pleuro- and hypo-centra found 

 in the same region in such fishes, for example, as Amia. But 

 there is no sign of actual centra ; the notochord evidently per- 

 sisted complete. The interpretation of wdiat is seen of the 

 vertebral column is not easy. In the segments close behind 

 the head it rather strongly suggests the vertebral column of the 

 sturgeon. It appears as though in the anterior segments the 

 neural spines may have been separate, but further back they 

 were almost certainly fused with the arches. The spines below 

 the dorsal fin-supports are markedly widened from front to back, 

 a peculiar feature, which is confii'med, however, by all the speci- 

 mens in which these parts are shown. In the anterior caudal 

 region the spines seem to be wide and short ; further back they 

 are narrower again and are lengthened to support the dorsal 

 ridge-scales. Their continuation into the tail-lobe is not shown 

 in any of the specimens. Well ossified ribs appear in many 

 specimens in the anterior abdominal region (cf. text-fig. 7), 

 extending more than half-way from the level of the lateral 

 line to the ventral border ; but in the posterior abdominal 

 region they are very short. They are succeeded by four or five 

 fairly long haemal spines, inclined strongly backwards across the 

 upper ends of the anal fin-supports. In the posterior part of 

 the tail-shaft the haemal spines, like the corresponding neural 

 spines, are lengthened to support the ventral ridge-scales, and 

 the succeeding spines serve to support the caudal fin-rays in the 

 usual way. The supports of the dorsal and anal fins are well 

 shown. There is one to each fin-ray. At the front of the dorsal 

 fin there is also a separate support for each of the fulcia that stand 

 in series with the rays, but the corresponding fulcra of the anal 

 fin, which arj more crowded, have only one support ior them all. 

 The distal ends of all the supports are notched or forked ; in the 

 case of the dorsal fin the two heads so formed appear to share in 

 the support of two adjacent rays. The internal ends are more or 

 less expanded, and seem to come into close contact with the 

 neural or hsemal spines, except perhaps in the case of those from 

 the back of the fin. The great proportionate size of the anal fin- 

 supports is a peculiar feature of the internal skeleton of 

 Ace7itrophorus. 



The Species of Acentrophorus. 



There are known at present three indisputable species of 

 Acentrophorus : A. varians and A. alius (Kirkby sps.) fi-om the 

 Upper Magnesian Limestone, and A. glaphyrus (Agassiz) from the 

 Marl Slate. A fourth species, A. ahbsi (Kirkby), appears to be 



Proc. Zool. Soc— 1923, No. III. 3 



