96 PISCES. 



ridge-scales form a prominent crest, and the narrow overlapped margin 

 of the other scales is not produced at the angles. The marginal teeth are 

 slender, conical, and somewhat spaced, the inner teeth are stouter. The 

 paired fins are small, the dorsal fin large, and this arises at or behind the 

 middle of the back to be in part opposed to the relatively small anal ; the 

 caudal fin is symmetrical and slightly forked. The typical species is 

 S. bergeri from the Upper Trias (Keuper) of Coburg and Thuringia, and 

 attains a length of about 0'18 m. Smaller forms also occur in the same 

 sandstones in Wiirtemberg and Warwickshire ; and one is known from 

 the Lower Trias (Bunter) of Alsace. Large species are well known in 

 addition from the Upper Karoo Formation (Stormberg Beds) of the 

 Orange Free State, South Africa, and from the black Triassic flagstones 

 of Massachusetts, Connecticut, and New Jersey, U.S.A. The American 

 species are commonly described under the generic name of hchypterus. 



Acentrophorus. Seems to differ from Semionotus only in the absence 

 of a prominent series of median dorsal scales. The typical species 

 (A. varians), nearly a centimetre in length, occurs in the Upper Permian 

 (Magnesian Limestone) of Fulwell Hill, near Sunderland. Other small 

 forms are met with in the Marl Slate of Durham and Northumberland 

 and in the Kupferschiefer of Saxony ; while more doubtful specimens are 

 recorded from the Triassic of Massachusetts and the Lower Permian of 

 Bohemia. 



Aphnelepis. A genus scarcely differing from Semionotus except in its 

 very thin scales, which are much the thinnest on the caudal region, and 

 do not form a conspicuous dorsal ridge. Known only from the Lower 

 Jurassic of New South Wales (A . australis from Talbralgar). 



Lepidotus (fig. 69). One of the best-known genera of Semionotida>, 

 with numerous species and having a very wide range both in space and 

 time. The trunk is fusiform, only moderately compressed, and entirely 

 covered with thick rhombic scales, of which the wide overlapped margin is 

 produced into a forwardly-directed point at each angle. The chondro- 

 cranium is well ossified, and the basicranial axis is straight. The lower 

 aspect, of the basioccipital is marked by a broad longitudinal groove, with 

 a flattened rim on each margin, probably indicating the backward 

 extension of the basicranial canal (for the recti muscles of the eye) as far 

 as the occiput. The exoccipital forms a considerable part of the lateral 

 wall of the brain-case, and the pro-otic is still larger. The ethmoids 

 surround a pair of long closed canals for the passage of the olfactory 

 nerves. The membrane-bones of the cranial roof form a continuous 

 shield, and are disposed as in Amiu. The cheek is also completely 

 covered, there being one circumorbital ring of plates, a semicircle of 

 postorbitals, and a short deep series of preorbital plates. The maxilla is 

 very delicate, deepest behind, tapering forwards, and terminating in front 

 in an inwardly-directed process ; its margin bears a series of styliform 

 teeth. The prernaxilla is a smaller, stouter bone, also with a single series 



