68 DR. W. G. RIDEWOOD ON THE CRANIAL [May 3, 



A very definite relation exists between tlie supratemporal and 

 pi-eopercular 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 con- 

 tinued 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 pre- 

 opercular*, 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 difierent 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 difierent 

 nature from the other opercular bones. On page 177 they classify 

 the preopercular with the lachrymal, nasal, suborbital and supra- 

 temporal bones, a step which I am fully prepared to endorse. 



The determination of the morphological value of the intei-- 

 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. Howevei', on the separation of the 

 preopercular from the opei'cular and bi-anchiostegal 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 ray. 



The ligamentous connection so commonly met with between 

 the front of the interopercular and tlie back of the mandible is 

 probably merely adaptive, and does not point to any morpho- 

 logical relation existing between this bone and the mandibular 

 arch. The question is discussed by Gegenbaur (Morph. Jahrb. iv. 

 Snppl. 1878, pp. 15 & 16, footnote). 



* To be strictlj' 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." 



