SKELETON OF EEGALECUiS AEGENTEUS. 25 



maximum length in the second dorsal fin is 3 inches, but this is reduced to 1| inch at 

 the anterior and posterior ends of the fin. 



The first fifteen rays are, as mentioned in the systematic description (p. 5), modified to 

 support the characteristic crest (figs. 1, 5, and 20). In the specimen described, only the 

 seventh and ninth of these are perfect, and measure 17^ and 15 J inches respectively. 

 The first to fifth rays have evidently lost very little; the first is 17^ inches long. All 

 these rays are semitransparent, not longitudinally divided, very brittle, and taper from 

 the proximal to the distal end, terminating when perfect in an extremely fine point 

 (figs. 2 and 3). 



The interspinous bones supporting the crest are also peculiarly modified (fig. 20). 

 The first nine of them come altogether in front of the atlas, and are gradually, passing 

 from behind forwards, more and more sloped forwards at their anterior ends, so that the 

 anterior five of them (isp. \-isp. 5) are nearer horizontal than vertical. These first five 

 interspinous bones are, moreover, ankylosed together, forming an irregular laterally 

 compressed bone, and thus affording a firm support to the crest. The dorsal end of 

 this compound bone shows clearly the extremities of the five interspinous bones which 

 enter into its composition ; its ventral (posterior) end is forked vertically ; the lower 

 limb of the fork is straight, abuts against the spine of the atlas a little below its middle, 

 is produced into two lateral wings so as to have a somewhat flattened lower surface, 

 and appears to belong to the first interspinous bone only ; the upper limb is curved, 

 nearly horizontal, connected with the extremity of the spine of the atlas, and apparently 

 belongs to the coalesced second to fifth interspinous bones ; with its upper edge the lower 

 ends of the sixth to tenth interspinous bones are connected. 



As shown in the figure of the entire skeleton (fig. 5) these peculiarly modified 

 anterior interspinous bones overhang the skull, and it is by the filling up of the interval 

 between the two that the " forehead " of Megalecus is produced. 



The articulation of the anterior rays presents some peculiarities. The first {fir. 1) 

 articulates with the cartilaginous extremity of the first interspinous bone ; the second 

 and third with a small cartilaginous pad, capping the posterior half of the first and the 

 second interspinous bones; the fourth, fifth, and sixth with a much larger pad applied 

 to the tops of the third, fourth, fifth, and anterior half of the sixth interspinous bones. 

 The remaining rays articulate in the normal manner with cartilaginous nodules 

 alternating with the interspinous bones. A slight variation is, however, seen as far as 

 the eleventh ray, in that the ligamentous fibres joining the interspinous bones take on 

 an increased development and form large pads of fibrous tissue (fig. 20, fp), immediately 

 underlying the cartilaginous nodules and capping the contiguous halves of two adjacent 

 interspinous bones. 



vol. xu. — part i. No. 4. — February, 1886. 



