Seven new Species of Testacea. 329 



Fig. 10. Testa jioiior, colore extus et intus pallidior, tenuior, 

 subdiaphana. 



I have no particular remarks to make on this species, except 

 that it has a good deal of the habit of a Mactra. Having no 

 striking character, as to either its figure or colour, it is very 

 liable to be confounded with some other species, though I have 

 endeavoured to describe it with precision; and, had the describers 

 of those shells which are most nearly allied to it been less ambi- 

 guous in their definitions, I should not fear that there would be 

 any mistake in referring to its name. 



4. Mytilus membuanaceus. 

 Tab. XXIV. Fig. 11, 12. 



Mytilus testa subrhombea, fragillima, margine anteriore an- 

 gulata. 



Habitat in America australi, fluviatilis. 



Testa subdiaphana, 1 poll, longa et lata, subventricosa, fer^ 

 membranacea, intus subniargaritacea, glaberrima, transversitn 

 delicatissim^ striata, colore extus viridescente, figura fer^ Mt/<re 

 variabilis senioris. Margo ad cardinem rectissimus. Cardo eden- 

 tulus. Unibones acuti. 



I have given the trivial name of memhranaceus to this Mytilus,, 

 on account of its extremely thin and tender texture, which forms 

 its most obvious character. The contour approaches so nearly 

 to that of Mya variabilis in its perfect stale, as to render it de- 

 sirable that they should both be placed in the same genus, did 

 not the hinges so materially differ: in fact, many of the My(B 

 and Mytili belong to one natural fan)ily, and there is often much 

 difficulty in determining under which name a particular species 



ought 



