﻿152 



SUPPLEMENT TO THE CRAG MOLLUSCA. 



Rose's collection from the Nar Valley Brickearth, and it is mentioned by Mr. Seeley 

 (under the name of T. proximo) as occurring in the gravel of March, though Mr. Harmer, 

 who has searched the gravel of that place very assiduously, has not been able to meet 

 with it. 



The shell which I have called lata was figured by Lister, and called by him Tellina 

 lata alba, and the name lata was adopted by Gmelin. This is the oldest notice of the 

 shell. The late Edward Forbes, 'Mem. Geol. Sur.,' 1846, p. 412, considered T. obliqua 

 as merely a variety of Lister's species from the seas of Norway. Although I have the 

 highest respect for the opinion of the late E. Eorbes (whose premature loss all geologists 

 deplore) I must in this instance dissent from it, notwithstanding that it has been adopted 

 by Mr. Jeffreys. Similarly E. Forbes regarded T. prcetenuis as merely a variety of this 

 shell, and Mr. Jeffreys has followed him. The word variety as distinguished from species 

 is, to my apprehension, too vague to carry any precise meaning with it, but as regards 

 these three forms, lata, obliqua, and prcetenuis, there can be no question as to their 

 complete distinction. T. obliqua is the only one of the three forms found in 

 the Cor. Crag, but in the Red Crag lying between the Stour and the Aide we get this 

 shell and T. prcetenuis abounding together, and T. lata very rare, while in the Fluvio- 

 marine Crag and Chillesford bed we get all three forms in abundance and well marked. 

 I am sure that all collectors from the Crag will bear me out in saying that the three 

 forms thus occurring in profusion together can be without difficulty selected, and that 

 they do not form merely the terms of an undistinguishable series. They were therefore 

 three distinct forms living in the same sea, and not intermingling ; and what else consti- 

 tutes species ? Two of them are, so far as I am aware, not known living, for nothing 

 recent that I have seen can justly be identified with either obliqua or prcetenuis. In the 

 first formation upwards from the Crag, viz. the Lower Glacial sands, T. prcetenuis becomes 

 rare, while obliqua maintains itself there in great profusion. As w T e ascend in the order 

 of formations we lose the form prcetenuis altogether, but still meet with obliqua in the 

 Middle Glacial sands and at Bridlington, but it has not yet been met with in any Post- 

 glacial bed, or in any of the English Newer Glacial, or in any of the Scotch beds ; while 

 lata not only occurs in these beds, but survives as an Arctic shell. The history of these 

 most important shells (for it is the most abundant species that are the most important in 

 a geological point of view) is thus clearly traceable in time through the various formations 

 much in the same way as I have traced that of the common Trophon antiquus. 



Abra alba, W. Wood. Crag Moll., vol. ii, p. 237, Tab. XXII, fig. 10. 



Localities. Cor. Crag, Sutton. Red Crag, Walton, Sutton, Bawdsey. Fluvio-marine 

 Crag, Thorpe, by Norwich ? Bulchamp ? Chillesford bed, Chillesford ? Aldeby ? Post- 

 glacial, Nar Brickearth, Pentney (Rose). 



