OCR Text |
Show A very definite relation exists between the supratemporal and preopercular bones. Disregarding a few exceptional cases, the supratemporal is a bone which is distinguished by a triradiate sensory canal, as already pointed out. The posterior tube is continued back into the post-temporal, the upper tube runs over the cranium, usually in or over the parietal bones, as the transverse commissure, while the antero-ventral tube, after traversing the superficial part of the squamosal, passes down into the preopercular*, either directly or through a subtemporal bone. The opercular and subopercular bones and the branchiostegal rays, on the other hand, carry no sensory canal, but are ossifications in the movable flap or gill-cover that projects back from the hyoid arch, and are thus bones of a different category. This exclusion of the preopercular from the opercular series is not a new idea, although independently arrived at, for it is to be noted that Cole and Johnstone (Proc. & Trans. Liverpool Biol. Soc. xvi. 1902, p. 175), in describing the osteology of the Plaice, have pointed out that the preopercular is a bone developed primarily around a portion of the lateral-line system, and is therefore of a different nature from the other opercular bones. On page 177 they classify the preopercular with the lachrymal, nasal, suborbital and supratemporal bones, a step which I am fully prepared to endorse. The determination of the moi'phological value of the inter-opercular bone is not an easy matter. On examining the skull of Amia it is difficult to believe that the interopercular does not belong to the same series as the opercular and subopercular bones, and yet in Lepidosteus the interopercular is quite removed from the subopercular, and is situated at the front of the lower or horizontal limb of the preopercular bone. Possibly the name interopercular has been applied in different fishes to bones of different morphological value. However, on the separation of the preopercular from the opercular and branchiostegal series of bones and the introduction of it into the temporal series, it becomes necessary to decide whether the interopercular belongs to the one or the other of these groups, and on the whole I am disposed to regard it as more closely related to the preopercular than to the subopercular and opercular bones. This is not the view taken by Cole and Johnstone, who consider (Proc. & Trans. Liverp. Biol. Soc. xvi. 1902, p. 175) that the not infrequent articulation between the interopercular and the epihyal confirms the view that the interopercular, like the opercular, is a modified branchiostegal rav. The ligamentous connection so commonly met with between the front of the interopercular and the back of the mandible is probably merely adaptive, and does not point to any morphological relation existing between this bone and the mandibular arch. The question is discussed by Gegenbaur (Morph. Jahrb. iv. Suppl. 1878, pp. 15 & 16, footnote). * To be strictly logical, such bones as the squamosal should also "be classed as sensory-canal bones of the temporal series; but it is clearly more expedient to regard them as component parts of the " cranium." 6 8 DR. W. G. RIDEWOOD ON THE CRANIAL [M a y 3 , |