104 ANATOMY OF VERTEBRATES. 



times (Diodon, Lopkius, Anguilla) exclusively suspended therefrom. 

 In the Lophius piscatorius the opercular is a long and strong bone 

 suspended vertically from the convex epitympanic condyle, and 

 with a long and slender fin-ray proceeding from the back part of 

 that joint. The subopercular forms the chief part of the opercular 

 fin by its long backwardly produced lower angle. The sub- 

 opercular bone in the Conger is soon reduced to a mere ray, 

 which curves backwards and upwards like one of the branchio- 

 stegals. The opercular itself, though shorter and retaining more 

 of its laminated form, also shows plainly, by its length and curva- 

 ture in the Eels, its essential nature as a metamorphosed ray of 

 the tympanic fin. We have seen that all the framework of this 

 fin had the forni of rays in the Plagiostomes. In Muraena the 

 small opercular bones articulate only to the under half of the 

 tympanic pedicle. The subopercular is wanting in the Shad. 

 The lowermost bone, called the interopercular, figs. 75, 84, 37, is 

 articulated to the preopercular above, to the subopercular behind, 

 and usually to the back part of the mandible ; it is attached, also, 

 in the Cod, by ligament to the ceratohyoid in front. The inter- 

 opercular and preopercular are the parts of the appendage which 

 are most elongated in the peculiarly lengthened head of the 

 Fistularia. 



The third inverted arch of the skull is the ' hyoidean,' fig. 81,38-41, 

 and is suspended, in Osseous Fishes, through the medium of the epi- 

 tympanic bone, 25, to the mastoid, 8 ; showing it to be the haemal arch 

 of the parietal vertebra. The first portion of the arch, stylohyal, fig. 

 85, 38, is a slender styliform bone, which is attached at the upper 

 end by ligament to the inner side of the epitympanic, close to its 

 junction with the mesotympanic, and at the lower end to the 

 apex of a triangular plate of bone, which forms the upper portion 

 of the ' great cornu.' I apply to this second piece, which is pretty 

 constant in fishes, the name of epihyal, ib. 39 : the third longer 

 and stronger piece is the ceratohyal, ib. 40. The keystone or 

 body of the inverted hyoid arch is formed by two small subcubical 

 bones on each side, the basihyals, ib. 41. These complete the 

 bony arch in some fishes : in most others there is a median 

 styhform ossicle, extended forward from the basihyal symphysis 

 into the substance of the tongue, called the glosso-hyal, ib. 42 ; and 

 another symmetrical, but usually triangular, compressed bone, 

 which expands as it extends backwards, in the middle line, from 

 the basihyals ; this is the urohyal, ib. and fig. 75, 43. It is connected 

 with the symphysis of the coracoids, which closes below the fourth 

 of the cranial inverted arches, and it thus forms the isthmus which 



