70 MYIDiE. 



shell is wedge-like on tlie longer side, with the terminal 

 edges reflected outwards : until it arrives at maturity 

 the truncation is incomplete. This alteration of shape 

 is not caused by absorption, but by the formation of 

 additional layers in fi-ont, which make the shell propor- 

 tionally longer or deeper than it previously was. The 

 Arctic variety, to which Forbes gave the name of Udde- 

 vallensis, is the usual form in glacial deposits ; it is 

 more depressed in the middle, obliquely truncated in- 

 wards, and excavated at the posterior end, frequently 

 to so great an extent, and in such a fashion, as if there 

 were cut 



"A huge half-moon, a monstrous cantle out." 



The internal structure of the shell is distinctly seen in 

 fossil specimens of this variety which have been perfo- 

 rated by the Cliona. A section thus exposed shows at 

 least 18 layers, and is unequally eroded, so as to re- 

 semble in miniature a perpendicular rock of oolite with 

 caverns at its base. A specimen of an intermediate 

 form, which I lately dredged in Dourie voe, Shetland, 

 measures 3^ inches in breadth, and is of proportionate 

 length. 



Petiver called this shell " The broad Pholade-muscle^'; 

 when half-grown it is the M, ovalis of Turton, and M, 

 pullus of S. Wood; the young is the Sphenia Swainsoni 

 of Turton, and M. Swainsonii of Loven. 



3. M. Bingha'mi*, Turton. 



Sphenia Binghami, Turt. Dith. p. 36, t. 3. f. 4, 5, and 1. 19. f. 3. Sphsnia 

 Binghami, F. & H. i. p. 190, pi. ix. f. 1-3, and (animal) pi. T. f. 3. 



Body elongated and compressed, pale yellowish-white : tubes 

 short, especially the incurrent one ; mouth of each encircled 

 by 5-10 rough white cirri ; valve of excretal tube large and 



* Named after the late Lieut. -General Bingham, an assiduous collector 

 of British shells. 



