Seven new Species of Testacea. 329 
Fig. 10. Testa junior, colore extus et intus pallidior, tenuior, - 
subdiaphana. : we 
I have no particular remarks to make on this species, except er 
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. | 
. MYTILUS MEMBRANACEUS. 
Tas. XXIV. Fig. 11, 12. 
-~ Myrtus testa subrhombea, fragillima, margine anteriore an- 
gulata. 
Habitat in America australi, fluviatilis. 
Testa subdiaphana, 1 poll. longa et lata, subventricosa, feré 
membranacea, intus submargaritacea, glaberrima, transversim 
delicatissimé striata, colore extus viridescente, figura fere Mye 
variabilis senioris. Margo ad cardinem rectissimus. Cardo eden- 
tulus. Umbones acuti. 
I have given the trivial name of ran 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 state, 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 Mye 
and Mytili belong to one natural family, and there is often much 
difficulty in determining under which name a particular species 
ought 
