﻿17 



55 



hind margin ol' this is joined lo the front margin of tlie lilade bl' of 4 (a similar 

 condition is also found in many other bony fishes between more or fewer, some- 

 times all the interspinous bones); on the other hand, the hind margin of the 

 posterior pari of the 4th interspinous bone is not in contact with any following 

 intersphial. In the remaining, distal part the usual interspinal form is now greatly 

 altered; the longitudinal keels (i.e. the continuation of the rod-like proximal pari) 

 extend out in both into the skin and there broaden out so as to be mutually in 

 contact and form the previously mentioned longitudinal suture (in scutata this 

 disappears through fusion); on 4, further, the broadened part bends round and 

 fuses with the hind (ventral) margin of the blade bl. In this way the bed both 

 for the anterior and posterior muscles of the spinous ray which is articulated to 

 the dorsal spine is covered over. The dorsal spine thus comes to enclose two 

 canals, an upper (anterior) and a lower (posterior) on each side; these canals are 

 very narrow but contain in the greater part of their length only the long tliin 



Fig. 5. 

 Amphisile scutata. Cross-section 

 through the dorsal spine. 3: third 

 interspinous bone, b its cartilagi- 

 nous axis; i: fourth interspinous 

 bone, b' its cartilaginous axis; o: 

 canal for tendon of M. erector, n 

 for M. depressor; s: canal for lateral 

 line. 



Fig. 6. 

 Centrisciis scolopax. Cross-section 

 of the upper ends of interspinous 

 bones 3 and ^i. Letters as in tig. 5. 

 Dotted lines indicate circumference 

 of the parts which would bring 

 about agreement with the condition 

 in Amphisile. 3 and //, Cross-section 

 of longitudinal keels on the 3rd and 

 4th interspinou,sgbones. 



tendons of the respective muscles; the muscles themselves chiefly occupy the space 

 indicated by a and b (which is covered over outwardly by the dermal armour) and 

 extend only a short distance into the beginning of the canals. On transparent 

 specimens oi A. strigata the muscles can be seen quite distinctly without preparation 

 under the exoskeleton. The cross-section (fig. 5) will possibly show the main cha- 

 racteristics better than any further description. It shows how the upper canal (o) 

 is formed by both of these interspinous bones, whilst the lower (n) is only formed 

 by the 4th. Fig. 6 shows for comparison a cross-section through the upper part of 

 the two corresponding interspinous bones in Centriscus scolopax, and gives an indi- 

 cation of the bony parts which could produce the condition characteristic for 



D. K. I) Vidensk. Selsk. Skr 



