422 MACTRIDiE. 



and more than three quarters of an inch thick. The 

 young and fry of this species have the same constant 

 characters as the adult. 



It is the M. stultorum of Pennant and M. lactea of 

 Poli (but neither of them Linne's species of those names), 

 M. triangula of Renier, M. cuneata of Sowerby's c Mi- 

 neral Conchology/ M. deltoides of Lamarck, M. Euxi- 

 nica of Krynicki (according to Middendorff), and the 

 M. obtruncata and M. triangulata of Searles Wood. 

 M. lateralis, Say (a common North- American shell), is 

 probably a variety or offset of the same species. 



B. Shell longitudinally rayed ; lateral teeth and sockets 

 smooth. 



3. M. stulto'rum *, Linne. 



M. stultorum, Linn. Syst. Nat. p. 1126 ; F. & H. i. p. 3G2, pi. xxii. f. 4, G, 

 and xxvi. f. 2. 



Body white faintly tinted with blue : mantle open through- 

 out the ventral range, thickened at its edges, which are clothed 

 with a delicate fringe of short intensely white filaments : tubes 

 never exsertcd more than half an inch, covered with a slight 

 and pale-brown wrinkled epidermis, which appears to be in- 

 dependent of that on the shell ; orifices encircled by dingy pale- 

 red unequal cirri, the alimentary tube having 12-16 (which 

 are somewhat longer than the other set), and the excretal tube 

 16-20 of these appendages ; the retractile valve of the last- 

 mentioned tube is conspicuous : gills narrow and attached for 

 two-thirds of their length, of the same proportionate size as 

 in other species, and (as well as the palps) similarly striated : 

 foot white, thick, and plastic. 



Shell of the same shape as M. solida, but more evenly 

 convex and always ventricose, thin, although nearly opaque, 

 and glossy : sculpture, numerous delicate and minute irregular 

 concentric striae, which are apparently formed by laminar folds 

 of the epidermis and become somewhat coarser at the sides ; 

 occasional lines of growth are also observable : colour yellowish - 



* Fools'. 



