UPPER, OR FLINTY CHALK. 221 



fragment that could be procured was carefully examined and preserved, 

 and the result has ultimately exceeded my most sanguine expectations, 

 since not only the generic, but also the specific characters of the original 

 have been determined. 



The remains in question are composed of a delicate friable crust, and 

 when first collected are of a dark chocolate colour, inclining to black, 

 but become pale, and lose much of their beauty by exposure to the air. 

 The inner surface only, is seen in those specimens that are exposed by 

 fracture ; it is glossy, and covered with minute circular depressions, formed 

 by the bases of the spines. The external surface is armed with short 

 spines, and papillee, and is invariably concealed by the chalk, until the 

 latter be carefully removed by art ; a process, which from the dehcacy of 

 the fossil, and the hardness of the surrounding matrix, is exceedingly 

 diflScult and tedious, and can scarcely be accomplished by an inexperienced 

 hand. Of the specimens in my collection, some contain the claws, 

 others the thorax, and a few exhibit the abdomen, and tail. These 

 detached parts having been accurately delineated, a restored outline of 

 the original was formed, and by a careful comparison of the latter with 

 the recent Crustacea, the genus and species of the original have been 

 ascertained. 



94. Astacus Leachii. Tab. xxix. figs. 1, 4, 5. Tab. xxx. figs. 1, 2. Tab. 

 xxxi. 



Gen. Char. Antennas pedunculated, unequal ; the exterior ones long 

 and setaceous; inner pair divided at the extremity; body elongated; legs 

 commonly ten ; tail foliaceous. 



Spec. Char. Thorax scabrous, convex, sixlobed, marginate; head semi- 

 circular in front; hands chelate, muricated, twice the length of the 

 thorax ; pincers very long, armed with obtuse spines. 



The thorax is longitudinally oblong, convex, covered with small 

 tubercles and papilla; it is divided into six lobes by a rounded dorsal 

 ridge, and two lateral sulci; the margin is entire. 



The form of the head of this species is unknown ; it appears to have 

 been semicircular or rounded in front, and is not distinct from the thorax. 



