Avicula. 



excessively prolonged. The left hand valve is contracted 

 and notched at the posterior side near the hase ; this is 

 also the case with the right hand valve but not nearly 

 in so eminent a degree: this sinus or notch is for the 

 passage of the byssus by which they fix themselves. 

 There is commonly a single small tooth in each valve, 

 just within the 2(mbo7ics: contrary to. fvhat Lamarck has 

 stated, we observe this little tooth very frequently in 

 his MeleogrincE, particularly when in a young state, and 

 we may also remark it is not always to be found in his 

 Avicuhe. The ligamental area is marginal, linear, nar- 

 row, dilated in the center ; (in old shells this dilatation 

 is very conspicuous and forms an obliquely trigonal 

 pit, commencing immediately below the umbones, and 

 gradually increasing in width towards the center of the 

 shell) ; and this is not peculiar to the Meleagrinae, but 

 also strongly characteristic of Avicula. There is another 

 circumstance which Lamarck dwells upon as evidencing 

 peculiarity of character in Meleagrina ; it is the rows of 

 imbricated scales on the outside ; but, unless these have 

 been worn off by attrition or the file, these are also com- 

 mon to his Aviculae; and we have represented one which, 

 has them produced into the form of longish flattened 

 spines. Muscular impression, nearly central, suborbicular^ 

 large, with a row of minute ones running from the inner 

 edge to the umbo. The external parts of the shell in both 

 consists of a foliaceous substance composed of pei'pendi- 

 cular fibres ; the internal of a brilliant pearly substance. 



The species of this marine Genus appear to be rather 

 numerous, though they were all placed in only two species 

 by Liime, his Mytilus Hinmdo and Margaritifenis, or the 

 Sivallow Muscle and the Pearl Muscle — the latter being' 

 the shell from which the Oriental pearls are obtained, 

 and being itself the substance commonly cd^eA Mother-of- 

 pearl. 



Of fossil species a few occur, particularly in the 

 London clay, and the strata identical with it. 



We have been obliged to give two plates in illustra- 

 tion of this Genus in which 



Fig. 1. Represents the two valves^of Avicula lieteroptera ? Lam. 



2. A specimen of Avicula aculeata; testa ohliqud, oblongd, aculeatd], 

 aculcis compressis, subimbricatis, confer tiusculis, siibdepressis; caudd longiorey 

 spiniferd. 



3. Avicula margaritifera, a small specimen j this shell sometimes grows 

 to a considerable magaitude, 



