94 MOLLUSCA FROM THE CRAG. 



Jordan Hall ; all the specimens I have seen from these Deposits are small, scarcely 

 exceeding half an inch in length, while the one from Bridlington has nearly twice 

 that diameter. 



This appears to differ but very slightly from the more elongated variety of 

 Z. caudata, considered a different species by our British Conchologists, as well as by 

 Dr. Loven, the principal difference is in the striae, which in this shell is finer or 

 more numerous. 



6. LEDA TRUNCATA, Brown. Tab. X, fig. 14, a 6. 



NUCULA TRUNCATA. Brown. Illust. Conch. Gr. Brit., pi. 25, fig. 19, 1827. 

 Smith. Mem. Wern. Soc., vol. viii, p. 42, 1838. 



Spec. Char. Testa transversd, ovatd ; concentrice striatd, tumidd, subcequilaterd ; 

 antice rotundatd, postice truncatd vel subsinuatd ; umbonibus prominulis ; margine 

 integerrimo. 



Shell transverse, ovate, subequilateral, tumid, concentrically striated ; anterior 

 side rounded, posterior truncate or subsinuated ; umbones slightly prominent ; margin 

 smooth. 



Length, Jths of an inch nearly. Height, \ an inch. 



Locality. ? 



This shell has been rejected by the authors of the ' Hist, of British Mollusca' as 

 an existing British species, and although not found in any of our Crag Deposits, 

 belongs undoubtedly to the Upper Tertiaries of England. It is an animal still living 

 in the Arctic Seas, but is no longer an inhabitant of our own. 



The specimen figured was obtained by Robert M'Andrew, Esq., who has 

 obligingly permitted me the use of it for the purpose of description : it was, he says, 

 in company with other, supposed extinct, species as Pecten Islandicus, &c., and 

 dredged from the depth of 40 to 60 fathoms, off the North Western Coast of the Isle of 

 Skye. It is ornamented with close-set strise, that appear independent of lines of growth, 

 as they occasionally bifurcate, and are not, therefore, quite parallel to the margin ; its 

 most peculiar character is on the posterior side, where there is a somewhat angular 

 ridge or keel from the umbo to the projecting beak-like termination, forming above 

 a large and elongate lunule-like space between it and the margin of the shell ; below 

 is another obtuse ridge extending from the umbo to the posterior part of the ventral 

 margin, and between this and the pointed termination, the shell is flattened or slightly 

 contracted at the margin, from which it is presumed it received its name : the shell is 

 tumid, the umbones somewhat curving over, so that the ligamental area projects 

 inwards ; there are from 12 to 18 teeth on each side, while the sinus in the mantle is 

 not very deep : the specimen is a full grown or aged individual, as the interior is 

 irregularly thickened, and more especially rugose between the margin of the shell 

 and the line of attachment by the mantle. 



