UPPER, OR FLINTY CHALK. 229 



of the gills is narrow, destitute of opercula, and placed above the 

 pectoral fins ; the branchiostegous membrane has two rays. The body 

 is compressed, and carinated on each side; the scales are coriaceous, joined 

 together, and rough, with sharp minute prickles. They have two dorsal 

 Jins, of which the anterior one is armed with a strong spinous ray, concealed 

 in a deep groove in the back, and can he erected or depressed by the animal, 

 at pleasure *. Some species, as the B. monoceros (Unicorn file-fish), are 

 furnished with a spine between the eyes. 



107. Spine of a species oi Balistes. Tab. xxxiii. figs. 5, 6. 



The specimen delineated in fig. 5, is evidently the defence of a fish, 

 and so strikingly resembles the spine fixed between the eyes of certain 

 species of Balistes, that there can be no hesitation in considering it as 

 belonging to a fish of that genus. It is of a dark chocolate colour, and 

 possesses a fine polish ; several vertebrae are imbedded in the chalk near 

 its base. 



Fig. 6, is nearly allied to the preceding, but in all probabiUty is re- 

 ferable to a different species of the same genus. 



Locality. Upper chalk, near Lewes. 



108. Dorsal fin, or radius, of a fish allied to the Balistes. Tab. xxxix. 

 This magnificent specimen, is one of the most interesting productions 



of the Upper chalk. It was unfortunately broken by the quarry-men, and 

 the intermediate portion destroyed: the dotted outline will, however, 

 assist in conveying an idea of its original form. 



It consists of thirteen narrow parallel rays, divided by fine sulci, that 

 gradually diminish in size as they approach the apex, which is broken off! 

 The rays are anchylosed, or united to each other, the grooves or furrows 

 penetrating but a short distance in the substance of the fin. The upper 

 edge is serrated, having fifteen obtuse projections, with corresponding 



* "lis ont deux nageoires aorsales, dont la premiere presente toujours un rayon tres fort, 

 et souvent garni d'epines, qui, couche dans tine fossette creusee dans le dos, pent se relever a 

 la volonte de ranimal, avec autant de vivacite que la corde d'une arbaletre qui se detend, ce qui 

 ne permet pas aux poissons voraces de les saisir, ou leur blesse gravement le palais lorsqu' ils 

 les ont saisis." Nouveau Diet. d'Hist. Nat. Tom. ii. p. 515. 



