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. 



2. MrA VARIABILIS. 



Tab. 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- 

 iiiide viridescente-fuscA, intus margaritacea, caerulescens, 1 poll, 

 longa (aetate provectd), vix 1 poll. lata. Margo antcrius suban- 

 gulatus, apud cardincm rectus. 



Testa junior minus gibba, subrhomboidea, fragilis, subdia- 

 phana, colore intus purpurascente, rugis mult6 prominentiori- 

 bus et fer^ 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 diflerence 

 of age. The most striking character in the younger specimens 



2 u 2 of 



