OSTEOLOG Y OF HEMIBRANCHIA TE FISHES-STARK,^. fi 2 5 



CEXTRISCOIDEA. 



Dermal armature connate with the internal skeleton, and developed 

 as a dorsal cuirass in connection with the neuropoph^'ses; six or 

 more anterior vertebrt^, extremely elong-ate, united: tail with its axis 

 deflected from that of the abdomen b}- encroachment of dorsal cuirass 

 over dorsal fin; branchial sj^stem feebly developed; parietals absent; 

 pterotics normally placed; condyle of basioccipital concave; myodome 

 absent; posttemporal united to cranium; supraclavicle present; fora- 

 men of hypercoracoid partly formed by clavicle; postclavicle present; 

 lower actinost elongated, cutting- into hypercoracoid; no open spaces 

 between actinosts; ribs developed; ventrals at middle of belly; spinous 

 dorsal developed below posterior s))ine of dorsal cuirass. 

 Family characters included above Centriscldse. 



THE SHOULDER GIRDLE IK DETAIL. 



THE "INTERCLAVICLES." 



The so-called intcrcknicles are here considered apart from the other 

 parts of the shoulder girdle for purposes of closer comparison. 



They may have been of different origin from the hypocoracoids, but 

 if so they have lost all trace of ever having been a separate ossifica- 

 tion. We can no more consider them as separate elements than we 

 can divide other bones which are of both cartilaginous and dermal 

 origin, and call each part by a different name. It is not true, as has 

 been supposed, that all of the members £>f the order Hemibranchii 

 have a differentiated part to the hypocoracoid. 



In Gastcmsten.'^ the part termed interclavicle by Parker/ as seen 

 externally, is the ganoid plate which bounds the lower edge of the 

 silvery area in front of pectoral. (A process from the clavicle bounds 

 it above.) It shows no sign of ever having been an ossification sepa- 

 rate from the hypocoracoid. It is attached to the clavicle above, arch- 

 ing away from it and attached again at its lower end, thus inclosing 

 an open space between. This is the typical arrangement of the hypo- 

 coracoid. The interclavicle may have been a plate of dermal bone 

 that has become fused with the cartilage bone of the hypocoracoid 

 beneath, but there seems to be no more necessity for giving it a sepa- 

 rate name than there is for giving a separate name to the ganoid proc- 

 ess from the clavicle. 



In Aulorhynchm the lower outer edge of the h^'pocoracoid turns 

 over slightly and forms the ganoid line which shows externally along 

 the lower part of the side in front of the pectoral. This can certainly 

 not be considered an interclavicle, and yet it differs but in degree from 

 the interclavicle of Gasterosteus. 



In Fistularki the interclavicle is the plate seen externally along the 

 side of the breast and belly. It is larger than in any other member 



* Structure and Development of the Shoulder Girdle, Ray Society, London, 1867. 

 Proc. N. M. vol, XXV— 02 40 



