BRACHIOPODA. 169 
within the mark than otherwise. Although large, this shell is somewhat thin in the 
brachial region, and the Coralline Crag specimens are generally either cracked or com- 
pressed simply by the weight of the matrix in which they are imbedded; but it becomes 
excessively thick by deposition of calcareous matter in old specimens, in the region of the 
viscera under the denticles of the larger valve. 
Mr. Charlesworth, in the ‘Mag. of Nat. Hist.,’ 1837, observed that the specimens of 
grandis found in the Red Crag up to that time were always separated valves, and it was 
inferred in consequence that these specimens were derived from an older formation. 
Specimens of this shell have, however, lately been turned out by the diggers for Coprolite 
in the Red Crag at Waldringfield in considerable numbers, many of which are in the 
possession of Mr. Charlesworth and Mr. Bell. These have the two valves united, and 
were probably inhabitants of the Red Crag Sea. hey differ in outline from the Coralline 
Crag sheil, being generally much more elongated and less regularly ovate or elliptical, 
having the brachial region more expanded, and the opening for byssus larger; but I 
believe they are all only a variety of this variable shell, well called 7. variabilis by 
Sowerby; and Mr, Davidson is of the same opinion. Specimens of these I have had 
figured as above referred to. This name is not in the list of “ Norwich Crag” shells by 
Woodward, neither have I seen it from any other locality of the Fluvio-marine Crag, or 
from the Red Crag of Butley. 
THREBRATULINA CAPUT-SERPENTIS, Linné. Supplement, Tab. XI, fig. 3 a—e. 
TEREBRATULINA CAPUT-SERPENTIS, Davidson. Brit. Tert. Brach., p. 12, 1852. 
\ Localities. Coralline Crag, Sutton. Recent, Britain, Mediterranean, &c. 
This is a very rare shell to my researches, and the few specimens that I have found 
are small and very young. ‘They nevertheless present a considerable amount of variation, 
both in the outward form and also in the number of rays or ribs with which they are 
ornamented ; one specimen, which measures one eighth of an inch in diameter, having only 
nine coarse large ribs, while another of the same dimensions has fifteen, and one yet 
smaller only seven. The form also is variable, being elliptical or much elongated in one 
specimen, while in another it is nearly orbicular. The byssal opening is also variable, but 
this probably depends upon the position m which the animal had chosen to fix itself. 
In one specimen the larger valve has a recurved beak like that of Rhynconella, while in 
another this opening is nearly rounded and sloping backwards. In these young shells 
there is sometimes a sort of incipient shoulder at the hinge-joint like that m the young 
state of 7. Gervillei, to which 1 had in my catalogue doubtfully referred the Crag shell, 
and the ribs are sometimes nodulous. 
This species is one of those which in the recent state is most remarkable for its 
