BIVALVIA. 113 



and stunted according to the nature of the habitat, changing its form in each stage of 

 growth from compressed and decidedly inequilateral, to very tumid and nearly 

 equilateral, even in the same locality, yet merging so imperceptibly into each other, 

 that they cannot be separated without dismembering what appears to me a good and 

 natural species ; and it is yet a doubt in my mind whether on a fuller investigation of 

 the genus, ' pusillum ' can continue to rank as a species." 



" These opinions have not been suggested by the sight of a few isolated and typical 

 forms, but after a long and patient examination of several hundred specimens, col- 

 lected in various localities in the counties of Berks, Cumberland, Devon, Dorset, 

 Essex, Hants, Herts, Kent, Lancashire, Middlesex, Northumberland, Surrey, Sussex, 

 and Yorkshire." 



In the form of my fossil specimens of pmillum, I can see no material difference 

 from obtusaJe, except that in the latter the shell is more tumid ; but the differences 

 between the two do not appear to me to be more evident than some of the forms are 

 in the fossil Ci/das cornea, where specimens are occasionally excessively tumid, while 

 others of the same length and height are much compressed, and the like differences 

 are observable in specimens of P. amnicmn. The few individuals of these last two 

 species, that I have seen from the Mam. Crag, are of the ordinary form or intermediate 

 between the extremes we have had figured. 



Lepton,* Ttcrton, 1822. 



SoLEN (sp.). Mont., 1803. 

 LuTRARiA (sp.). Gray, 1825. 

 PsAMMOBiA (sp.). Bioivn, 182/. 

 Erycina (sp.). Nyst, 1844. 



Generic Character. Shell equivalve, subequilateral, ovate, or subtrigonal, thin 

 and compressed ; umbones more or less acute, not prominent ; surface elegantly 

 ornamented ; margin plain ; hinge composed of two diverging teeth in each valve, 

 between which is placed the ligament wholly internal. Impression of the mantle 

 simple or without a sinus. 



The animal of this genus is said to have its mantle freely open in front with a fringe 

 all round the margin, and capable of extending itself considerably beyond the shell ; 

 a short siphonal tube with a single aperture, and a thick foot furnished with a byssal 

 groove ; one of the filaments of its marginal fringe is longer and larger than the others. 



In addition to the two recent British species, the Crag contains one quite distinct, with 

 another doubtful one resembling what appears to be a different species in the Campinian 

 beds of Belgium. Conrad also describes one living in the Seas of America, as well as 

 another from the Upper Tcrtiaries of that country, but few specimens of either of 



* Etym. AcTrros, thin. 



15 



