344 PEOF. T. W. BRIDGE ON THE MOEPHOLOGT OF THE SKULL 



extremity of the latter bone. The interoperculum (in Protopterus, " branchiostegal," 

 Owen, " preoperculum," Cobbold ; in Geratodus, " suboperculum," Giinther, " inter- 

 opercuhim," Huxley) is also a somewhat slender bone, shorter than the operculum, 

 with a hooked ventral or distal extremity, and situated directly behind and parallel to 

 the last-mentioned element (fig. 1, i.op.). The operculum is attached by strong liga- 

 mentous fibres to the hinder margin of the squamosal and the suspensorial cartilage, 

 while the interoperculum is similarly connected with the outer surface of the proximal 

 portion of the cerato-hyal, with the operculum, and also \vith the contiguous margins of 

 the squamosal and the suspensorial cartilage. A strong but nevertheless slender liga- 

 ment extends also from the ventral extremity of the interoperculum obliquely down- 

 ward and forward to its insertion into the inferior margin of the mandible. 



The interesting series of cartilages first described by Huxley [15] in connection with 

 the inner surface and distal extremity of the operculum and interoperculum of 

 Ceratodus are also represented in Lepidosiren, although apparently overlooked both by 

 Bischoff and Hyrtl. Closely applied to the inner surface of the interoperculum, and 

 considerably wider tlian the bone itself, is a thin lamina of cartilage (PL XXVIII. 

 figs. 1 and 4, v.liy.r.), the upper extremity of which is segmented off from the rest as 

 an apical piece and projects somewhat beyond the extremity of the bone. In the 

 operculum, however, the only indication of these structures is in the form of a thin 

 nodule of cartilage attached to the inner surface of the upper extremity of the bone and 

 projecting slightly beyond it (fig. 1, v.hy.r.). As Huxley (/. c.) has suggested in the 

 case of Ceratodvs, these cartilages may be regarded as vestiges of the cartilaginous 

 hyoidean rays of Elasmobranchs, and in the latter group are often arranged in two 

 series, a dorsal series attached to the hinder margin of the hyomandibular and repre- 

 sented in the Dipnoi by the opercular cartilages, and a ventral series connected with 

 the cerato-hyal, and having as their Dipnoid equivalents the interopercular cartilages. 

 The presence of vestigial cartilaginous hyoidean rays in conjunction with bony 

 opercular elements seems to be peculiar to the Dipnoi ; at any rate, after a careful 

 search I have failed to find any indication of their existence in those Fishes in which one 

 might with some reason expect to find them, viz. : — Acipenser, Polyodon, Polypterus, 

 and Amia. It is interesting to notice, however, that in Polyodon [3] the bony oper- 

 culum and interoperculum still retain the rayed character of their cartilaginous 

 predecessors. 



The branchial arches of Lepidosiren have been briefly described, but not figured, by 

 both Bischoff and Hyrtl. They are five in number (PI. XXIX. fig. 10), and separate 

 four linear branchial clefts. On each side the arches consist of a series of slender, 

 unsegmented, cartilaginous rods, distinct from one another and from their fellows of 

 the opposite side, and situated in the walls of the pharynx immediately external to the 

 pharyngeal mucous membrane. Dorsally and ventrally their extremities are con- 

 nected by a fibrous band ; but, apart from the fact that the dorsal band is looselv 



