534 PEOr. T. W. BEIBGE OK THE MESIAL 



segments of the radial elements, or of both proximal and mesial ; 

 (b) the suppression by fusion or atrophy of particular segments, 

 so that more or fewer of the elements become bisegmental instead 

 of trisegmental ; and (c) the apparently secondary subdivision of 

 the distal segments. 



The horny fibres which support the peripheral portions of the 

 fins are several times more numerous than the supporting radial 

 cartilages. 



HOLOCEPHALA. 



According to Mivart (I. c), the second dorsal fin of Callo- 

 rfiynclius antarcticiis is supported by a series of forty-one, not 

 quite contiguouis, simple and undivided radial elements, of which 

 the anterior are the longest, the remainder gradually decreasing 

 in length from before backwards. 



In a skeleton of Chimcera monstrosa in the Mason College 

 Zoological Museum there are about one hundred and two 

 similarly simple elements in the relatively much longer posterior 

 dorsal fin of this species. JSTone of the cartilages are in appo- 

 sition, all being separated to a greater or less extent, while at the 

 same time they are connected and supported by the longitudinal 

 fibrous septum separating the dorso-lateral muscles of opposite 

 sides of the body. As is well-known, the radial elements of the 

 anterior dorsal fin in both genera are greatly modified by con- 

 crescence and in other ways, for the support of the powerful 

 spine. 



GANOIDEI. 



ACIPENSEEID^. 



Acivenser sturio. 



As might be expected, the fin-supports of this and the next 

 species are essentially similar to those of the Elasmobranchs, 

 except for their partial ossification. 



Dorsal fin.— In Acipenser the dorsal fin is supported by a 

 series of sixteen distally distinct radial elements, ea,ch of which, 

 with the exception of the first two, consists of three segments, 

 the proximal being the longest, while the distal is reduced to 

 little more than a mere nodule. The first and second have appa- 

 rently lost their distal segments. The longest radial element is 

 the third, the first and second being somewhat shorter, while those 

 behind the third gradually diminish in length to the two or three 

 most posterior ones, which are by far the shortest of the series. 



