DAVIDSON — SCOTTISH CARBONIFEROUS BliACJIIOl'ODA. 
225 
Howood, near Paisley ; and Orcliard-quany, Thomlicbank, lu Kircudbri^ht- 
shire, in strata cropping ont on the sea-shore near Kircudbright. In Stirling- 
shire, in tlic Balglass Burn beds, and in those of the Canipsic main lime- 
stone, &c., and Con-ie Bnrn.* 
Family Discinid.-e. 
Genus Discina. Lamarck. 1819. 
The shells belonging to this genus are usually circular, or longitudinally oval ; 
the larger or imperforated valve being conical, or limpet-like, with the apex in- 
clining toM'ards the posterior margin. The lower valve is conical, opercular, 
flat, or partly convex, and perforated by a narrow, oval, longitudmal slit, whicli 
reaches to near the posterior margin, and which in recent species is placed in 
the middle of a depressed disc ; the shell being always attached to marine 
bodies by means of a pedicle, and never by the substance of its shell as in 
Crania. The valves are luiartieulated, and kept in place by a particular dis- 
position of muscles ; the occlusor and divaricator impressions bemg somewhat 
similarly situated to those of Crania. 
Much has stiU to be done before the animal will have been completely and 
satisfactorily anatomically iuvestirated. The oral arms liave been described by 
Mr. S. P. Woodward, in his excellent Mamial, as being curved backwards, re- 
tiu'ning upon tliemselves, and ending in smaU spires, directed downwards 
towards the ventral valve, as in the subjoined figure; and the only process 
In this diaoramrDatic representation of the animal of Discijia, by Jlr. ■Woodward, the cxfci-em- 
itdes of the labial arms ai-e displaced forwards, in order to show their spiral tennuiations. 
which could possibly have afforded support to the arms is developed from the 
centre of the ventral valve, as in Crania. In recent species the shell is stated 
by Dr. Carpenter to be horny and minutely punctate, the tubuli being generally 
arranged in fasieuli, so that their transverse section presents a series of dots. 
Dr. Gratiolet believes, however, that the shell is not entu-ely composed of a 
Lomy substance, but somewhat similar to that of Lingula, although the cal- 
careous element is enormously greater in the last-named genus. Tlie chemical 
composition of the shell of D'isciua has been stated by Monsieur S. Cloez to be 
similar to that of Lingula, of which an analysis will be found further on. 
* In Ireland C. qimdrata is found at Rahan'e Bay, in Donegal. Appenvs to be a rare shell 
m EnsUsh Carboniferous strata, a single example havmg been hitherto ioimd by Mi-. 
C. Moore, at HolweU, near Frome. It occurs also at Toumay, m Belgium, and at Tuscombia, 
Alabama, in America. It may Ukewise be obseri^ed that many species of Crama have been 
confoimded with Discina, from the fact that it is often very difBcult to distmguish certain fossil 
species when the interior cannot be examined. 
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