OCR Text |
Show 1904.] OSTEOLOGY OF THE ELOPID.E AND ALUULIDJ3. 69 t might reasonably be urged that the interopercular is a bone or the true opercular series which has been squeezed out of its primitive position between the subopercular and the last branchiostegal ray, in much the same way as the penultimate member of the branchiostegal-opercular series has been reduced and forced out of its position in the Palaeoniscid fishes. The bone in question was originally regarded by Traquair (" Ganoid Fishes Brit. Carb. Form., Palseontogr. Soc. 1877, p. 20) as the subopercular, but subsequently {ibid. 1901, p. 62) as an accessory plate. Smith Woodward appears to regard it as the interopercular (Brit. Mus. Oat. Foss. iish. ii. 1891, p. 487). But, whatever may be the homology of the bone, the evidence that it has been excalated and finally lost is tolerably clear. In Teleosteans, however, there is no evidence available to show that any such displacement of the interopercular has taken place; even in Pholidophorus, the lowrest of the Teleostean series, the interopercular already occupies its definitive position in advance of the subopercular. In that very aberrant genus Phractolcemus, the preopercular is small and the interopercular remarkably large. The latter bone is situated below and anterior to the preopercular, and receives from it the sensory canal that descends from the squamosal. The interopercular is thus here performing the function of the missing horizontal limb of the preopercular. In no other case have I seen the interopercular conveying a sensory canal; in the vast majority of cases the interopercular is a thin lamina of bone which is almost entirely concealed by the lower part of the preopercular. The evidence for putting the interopercular with the preopercular is unsatisfactory, but owing to the perfect manner in which the branchiostegal rays grade off into the subopercular and opercular, there is no justification for including it with these last as a constituent of the skeleton of the gill-cover. The interopercular is of very regular occurrence, but it is said to be wanting in Pantodon. As regards the preopercular, it may be taken as a general rule, not however without exception, that the horizontal limb of this bone is most developed in those forms with a greatly reduced mouth, e. g. Gonorhynchus and Chanos, and absent in those with a very large gape, e. g. Engraulis and Coilia. In Albula and Alepocephalus the vertical and horizontal limbs are nearly equal. Circumorbital and Nasal Series.-No results of any great morphological importance are to be expected from a comparative study of the bones of the circumorbital series. They are probably more subject to variation-generic, specific, and individual-than any other bones of the skull; and the number of the bones surrounding the orbit is not infrequently found to differ on the right and left sides of the same skull. In Arapaima, Heterotis, and Osteoglossum the enlargement of the nasal bones, their meeting in a median suture, and their rigid union with the cranial bones, are features which are not encountered in any of the other genera under examination. The |