Seven new Species of Testacea. 327 
length, and rather more than } an inch in width. It is one of 
the most remarkable bivalves with which I am acquainted, part 
of the margin of one of the valves projecting over the correspond- 
ing part of the other, so as, exactly, to resemble a lip. It is for- 
tunate when so striking a character presents itself, for the species 
cannot, in such circumstances, be mistaken. 
9. Mya VARIABILIS. 
Tas. XXIV. Fig. 4, 5, 6, 7. 
Mya testa subrhombea, gibbosa, umbonibus longitudinaliter 
corrugatis, cardinis dentibus duobus, apice divergentibus, foveis 
linearibus invicem insertis. 
Habitat in America australi, fluviatilis. 
Testa transversim striata, rugis sensim evanescentibus, epider- 
mide viridescente-fuscá, intus margaritacea, cserulescens, 1 poll. 
longa (atate provectä), vix 1 poll. lata. Margo anterius suban- 
gulatus, apud cardinem rectus. 
Testa junior minus gibba, subrhomboidea, fragilis, subdia- 
phana, colore intus purpurascente, rugis multó prominentiori- 
bus et ferè ad marginem usque divergentibus. 
This species varies extremely in its structure and contour, ac- 
cording to its several stages of growth; and, if I had seen only 
the youngest and the oldest shells, without having had oppor- 
tunities of comparing those of intermediate ages with each, I 
should most probably have given them separate places in the 
genus. ‘There can be no doubt that many other testacea (espe- 
cially in the genera of Mya and Mytilus), at present considered as 
distinct species, will, from the occurrence of similar opportuni- 
ties, be found to owe their difference of form solely to difference 
of age. The most striking character in the younger specimens 
2u2 of 
