[47] THE OSTEOLOGY OF AMIA CALVA. 793 



side of my specimen I find but one of them, wliicli is situated just be- 

 low the squamosal and shaped like the one marked W on the right side. 



All four of the opercular hones are present and thoroughly developed, 

 (Fig. 16, Ojy., S. 02>'-, I- op.^ and Pr. op.) 



The preoperculnm is a long, narrow, crescent-shaped bone, that touches 

 the squamosal above and contributes to the articulation for the mandi- 

 ble below. Only a narrow strip of its external surface, just within the 

 posterior border, along its entire length, shows the sculpturing commou 

 to the other bones. Beyond this its surface is smooth, and its anterior 

 border makes a very intimate union with the hyomandibular and sym- 

 plectic. 



The three remaining opercular bones are beautifully sculptured all 

 over their external surface, and remind one not a little of those rugose 

 surfaces as seen in some of the handsome marine shells. Of these bones 

 the operculum is by far the largest ; it articulates with an elongate facet, 

 placed upon the upper and posterior angle of the hyomandibular. In 

 common with the remaining two of the group, its anterior border is over- 

 lapped by the preoperculnm. The upper and lower margins of the sub- 

 operculum are closely applied throughout their entire extent to the op- 

 posed margins of the operculum and iuteroperculum. This element is of 

 a more irregular form than either of the others, its upper border being 

 deeply concave to admit the rounded lower anterior angle of the oper- 

 culum, while the inferior one is quite straight. Against this last, the 

 base of the interoperculum is applied, this latter plate having somewhat 

 the form of an isosceles triangle, with its rounded apex directed below. 

 The inner surfaces of these three last opercular bones are smooth and 

 unmarked by any sculpturing, as their opposite sides are. A rounded 

 ridge crosses the suboperculum obliquely, extending from its upper pos- 

 terior angle to the lower anterior one. Anteriorly, the extremity of the 

 maxillary (Fig. 16, Mx.) is bent towards the median line, and articu- 

 lates in a socket immediately behind the outer end of the premaxillary, 

 being covered over above by the preorbital and lacrymal plates. Its 

 entire lower margin is armed with a single row of thickly set teeth. 

 These decrease in size from before, backwards, and, like the larger ones 

 on the premaxillary are very sharp and gently curved inwards. The 

 hinder half of the upper border of the maxillary supports an additional 

 thin plate of bone, as seen in so many of the Teleostei. This is the ad- 

 maxillary, and its form is very much the same as in bony fishes (Fig. 

 16, a). Both the maxillary and admaxillary are sculptured on their 

 outer surfaces after the fashion of the other ganoid plates described 

 above. 



Bridge says : " In comparing the skull of Amia with the skulls of 

 certain of the Siluroidei, and notably with that of Clarias, it is inter- 

 esting to notice that, in addition to the more obvious and less impor- 

 tant points of resemblance between the two genera necessitated by 

 the flattened condition of the head and a foreshortening of the prefrontal 



