DAVIDSON — SCOTTISH CAEBONIFEEOUS BEACHIOPODA. 225 



Ho wood, near Paisley_ ; and Orcliard-quan-jj Thornliebank. _ In Kircudbri^lit- 

 shirCj in strata cropping out on the sea-shore near Kircudbright. In Stirlmg- 

 shire, in the Balgiass Bui-n beds, and in those of the Campsie main lime- 

 stone, &c., and Corrie Burn.* 



Eamly D1SCINID.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 towards the posterior margin. The lower valve is conical, opercular, 

 flat, or partly convex, and perforated by a narrow, oval, longitudinal slit, wliicli 

 rcciuhes to near the posterior margin, and wliich in recent species is placed in 

 the middle of a depressed disc ; the shell being always attached to marme 

 bodies by means of a pedicle, and never by the substance of its shell as in 

 Crania. The valves are unarticulated, and kept in place by a particular dis- 

 position of muscles ; the occlusor and divaricator impressions being somewhat 

 similarly situated to those of Crania. 



Much has still to be done before the animal will have been completely and 

 satisfactorily anatomically investigated. The oral arms have been described by 

 Mr. S. P. Woodward, in liis excellent Manual, as being curved backwards, re- 

 turning upon themselves, and ending in small spii'cs, directed downwards 

 towards the ventral valve, as in the subjoined figui'c ; and the only process 



In this diaOTammatic representation of the animal of Discina, by Mr. Woodward, the extrem- 

 ities of the labial arms are displaced forwards, in order to show their spu-al terminations. 



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 fasiculi, so that their transverse section presents a series of dots. 

 Dr. Gratiolet believes, however, that the shell is not entirely composed of a 

 homy substance, but somewhat similar to that of Lingula, although the cal- 

 careous element is enormouslv greater in the last-named genus. The chemical 

 composition of the shell of Discina 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. qiiadrata is fomid at Rahan's Bay, in Donegal. Appears to be a rare shell 

 in EnffUsh Carboniferous strata, a smgle example havmg been hitherto found by Mr. 

 C. Moore, at HolweU, near Frome. It occurs also at Touxnay, m Bel^um, and at Tuscombia, 

 Alabama, in America. It may hkewise be observed that many species of Crama have been 

 confounded with Discina, from the fact that it is often veiy difficult to distmgmsh certam fossil 

 species when the interior cannot be examined. 



VOL. HI. ^ 



