30 



NATURAL SCIENCE. 



tapering, rod-like supports is often still preserved between the lateral 

 muscles of the trunk. 



There is, however, another method of subdivision of the 

 median fins, specially characteristic of the Elasmobranchii, 

 Crossopterygii, Chimaeroidei, and, perhaps, some Dipnoi. At two 

 points in the dorsal region — and never at more or less than two 

 points, so far as known^ — there is a tendency towards the crowding 

 and concentration of the endoskeletal fin-supports, usually resulting 

 in the more or less complete fusion of the proximal parts that with- 

 stand the growth-pressure, accompanied by the varying fusion of the 

 distal remnants of the mass of supports. At one or two points in 

 the anal region, a similar phenomenon occurs. This is illustrated, to 



Fig. 2. — Abdominal vertebral segments of Plemacaiithus. ch. Notochord ; d. Neural arch; 

 h. Haemal arch ; a, b, c. The three series of dorsal tin-supports. After A. Fritsch. 



some extent, by the accompanying drawing of the anterior anal fin of 

 Xenacanthns (Fig. 3), in which two haemal arches (b, c) directly support 

 the crowded and modified cartilages. A more striking case, however, 

 occurs in the dorsal fin of the Devonian Crossopterygian Holoptychius 

 (Fig. 4), in which the basal portions of no less than seven supports 

 are fused into one mass, and remnants of a still greater number are 

 clustered together distally. A further stage in this specialisation is 

 met with in the anterior dorsal fin of the Crossopterygian Ccclacan- 

 thidae, where the completely fused triangular base alone remains, the 

 distal renmants of the supports are lost, and the dermal fin-rays are 



•^ The apparent exceptions oiNotidanus and Chlainydoselaclie require examination. 



