230 UPPER, OR FLINTY CHALK." 



depressions. The inferior margin is entire, and near the base of the fin, 

 is furnished with numerous slender processes, or cirri, that occupy a space 

 of three inches in length, and an inch and a half in breadth ; these are 

 probably the remains of the tendinous expansion of the muscles, by which 

 the fin was erected, and depressed. 



The external surface is in a great measure destroyed, but where 

 portions of it remain, exhibits a granulated structure, Hke seal-skin. 

 This appearance is distinctly seen near the two first teeth, and towards 

 the smaller extremity. The fin is soHd, and is composed of a brown 

 brittle substance, resembhng the constituent matter of the vertebrae of 

 cartilaginous fishes found in the chalk. 



It is 10-5 inches long ; 3-5 wide at the base ; and about half an inch 

 thick. 



In " Townsend's Character of Moses," a dorsal fin bearing a general 

 resemblance to the present fossil is figured, PI. xviii. figs. 1, 2, 3. 

 Similar specimens occur also in the Blue Lias of Dorsetshire. These fins 

 consist of parallel rays, and are armed on the upper margin with sharp 

 spines, placed in sockets, like the teeth in the proboscis of the Pristis. 

 It must however be remarked, that in the specimen under consideration, 

 the tooth-like processes are prolongations of the substance of the fin, and 

 not distinct processes ; a structure that separates it from any fossils pre- 

 viously discovered. It differs also from the dorsal fins of the EaHstes in 

 its form, in the greater number of rays, and in the latter being anchy- 

 losed ; while in the recent species the rays are but few, and placed at 

 some distance from each other, being united by a membrane. It is there- 

 fore obvious that the fossil in question belonged to a fish, of which the 

 recent prototype is either extinct, or unknown. 



Locahty. Upper chalk, near Lewes. 



109. Dorsal fin of a fish allied to the genus Balistes. Tab. xl. fig. 3. 



This fin is less perfect than the preceding, and differs from it in the 

 tooth-hke projections being larger, and more distant ; those of No. 107 

 being twice as numerous. There can, however, be no doubt that it 

 belonged to a different species of the same genus. 



