128 WEALDEN AND PURBECK FOSSIL FISHES. 



is inserted close to the ventral border. When adpressed to the trunk this fin 

 extends at least half-way to the insertion of the pelvic fins. The latter are 

 smaller than the pectorals, and their supports are a pair of elongated thin laminae, 

 which meet in the middle line and are thickened along their outer borders. The 

 dorsal fin arises well in front of the middle point between the occiput and the 

 caudal fin, comprising 18 to 20 rays, of which the three foremost are short, 

 undivided, closely pressed together, and gradually increase in length. The length 

 of the fourth or longest dorsal fin-ray somewhat exceeds two-thirds of the depth 

 of the trunk at its insertion. The anal fin resembles the dorsal, but is much 

 smaller, and comprises only ^'^ or 14 rays. The remains of the caudal fin-rays 

 show that they are comparatively stout and closely articulated. The fulcral scales 

 at the base of the upper caudal lobe are especially stout, and are continued up the 

 foremost ray as a short fringe. 



The scales are relatively large, cycloid, and smooth, occasionally with feeble 

 traces of a radiating pectination at the hinder border, but usually exhibiting 

 structural lines, including wavy concentric markings. They are scarcely displaced 

 in the fossil, and are seen to be deeply overlapping, with the exposed area narrow 

 and deep. The lateral line is scarcely traceable, but seems to produce a slight 

 depression along some scales in a series above the vertebral column. 



Remarks. — The fossil thus described evidently belongs either to Leptolepis or 

 to Mthalion, but the absence of the mandible leaves its reference to one or other 

 of these two genera uncertain. It was originally assigned to Leptolepis, but the 

 extent of the intermuscular bones and the close articulation of the caudal fin-rays 

 are suggestive rather of Mthalion ; its general resemblance to the species of 

 Mthalion described by Traquair from the Wealden of Bernissart, Belgium, is 

 indeed noteworthy. It may, therefore, best be placed provisionally in the latter 

 genus. Its number of vertebrae exceeds that of all the known species of both 

 Leptolepis and Mthalion, except A. vidali from the Upper Jurassic of Spain. 



Horizon mid Locality. — Weald Clay: Southwater, Sussex. 



Genus PACHYTHRISSOPS, novum. 



Parathrixxo})*, C. R. Eastman (non Sauvage, 1875), Mem. Carnegie Mus. Pittsburgh, vol. vi (1914), 



p. 423. 



Generic Characters. — Head as in Leptolepis, but the elevation of the dentary 

 relatively broad and less thickened. Vertebral centra much strengthened by 

 secondary ossification in fine longitudinal ridges, short and deep in the abdominal 

 region, sometimes longer in the tail. Pelvic fins relatively large ; dorsal and 

 anal fins acuminate, the dorsal about as long as deep, arising opposite or just 

 in advance of the origin of the anal fin, which is equally deep and usually more 



