70 xMONOGRAPH OP DUliA DEN. 



but in some specimens the lateral bones meet in the middle line in front of tlie 

 median one. The lateral bones have radiating strise on the posterior halves 

 of their outer surfaces. The anterior margin of each unites with the parietal 

 of its side, and then passes backwards and outwards, so as to form a large re- 

 entering angle with the postero-lateral edge of the parietal. Into this re- 

 entering angle, two bones, which would seem to represent the squamosal of 

 ordinary fishes, are fitted. A large, suborbitar bone extends from the post- frontal 

 to the maxilla, and forms the posterior boundary of the orbit. The pre-oper- 

 culum is a very considerable bone, which is attached to the parietal between 

 the squamosal and post-frontal ; to the latter bone and to the suborbitar bone 

 by its concave anterior margin ; to the representatives of the squamosal, and 

 to the operculum and sub-operculum, by its convex posterior margin, while 

 inferiorly it extends to the angle of the mouth. The operculum is a broad 

 bone, larger behind, where it is convex, than in front, where it is concave, and 

 much longer than it is deep. The inferior limits of the sub-operculum were 

 not distinct in any specimen I examined, but the bone appears to have been 

 deeper than long. 



The rami of the lower jaw are stout and strong, and form a very broad, 

 almost semicircular arch. Two broad, triangular, jugular plates occupy the 

 middle of their interval, and are separated from the rami by a number of 

 smaller bony plates on each side. There is no median plate between the 

 anterior ends of the principal jugular plates. 



The outer surfaces of the bones of the pectoral arch exhibit radiating ridges, 

 where they are not overlapped by the opercular bones. 



The characters of the scales are well known. Those of the lateral line 

 exhibit a slight median elevation, and form a series which descends from 

 close to the occipital region downwards and backwards upon the sides of the 

 body, whose axis it very nearly follows in the caudal region. 



A small triangular dorsal fin begins opposite the hinder edge of the root of 

 the ventral fin, and is situated a little behind the middle of the body. It is 

 separated by about the breadth of its own base from the commencement of 

 the dorsal lobe of the caudal fin,* which occupies nearly the posterior third 

 of the whole length of the body, and attains its greatest height about the 

 middle of its length. The caudal end of the body gradually tapers to a 

 point which is not at all bent upwards, and the ventral lobe of the caudal 

 fin, though rather shorter than the dorsal lobe, has the same depth. The 

 caudal fin consequently forms a very nearly symmetrical rhomboid, and is 

 not, in the ordinary sense, heterocercal. The anal fin is [^rather larger than 

 the dorsal, and is separated by but a veiy small interval from the ventral 

 lobe of the caudal. 



A wide space separates the ventral fins from the anal, the former being 

 situated just beneath the middle of the body. Each has a short, conical, 

 scaly stem, from either side of which long fin rays proceed. 



* Sir Philip Egerton, in a valuable memoir recently read before the Geological Society, 

 expresses his belief that Holoptychius has two dorsal fins. 1 am very loath to controvert the 

 opinion of so experienced and skilful an observer, the more particularly as specimens of Ho 

 lojjtycJdus with perfect tails are very rare, but the one or two complete examples I have 

 seen, leave no room in mv mind for any other conclusion than that stated above. 



