AVICULA. 



Avicula et Maleagrina^ Lam. 



TESTA extra plerumque sqiiamifera, nonnim- 

 quam siibmiitica, insequivalvis, rotuDclato-siib- 

 quadrata, basi transversa, recta, extremitatibiis 

 plerumque produetis, iiomimiquam brevibus, 

 antica interdum caudiformi: valva sinistra ad 

 basim angustata, emarginata. Cardo plerumque 

 dente in utraque valva infra umbones. Area 

 ligamenti marginalis, linearis, angusta, medio 

 dilatata. Impresslones muscular es piures, una 

 subcentralis, suborbicularis, magna, caeteree 

 minimis, in seriem interruptam ordinatse, um- 

 bonem versus deeurrentes. 



It is not without tlie strongest conviction of tlie propriety 

 of combining' the two Lamarckian genera Avimla and 

 Meleagrina in one, that we have taken our resolution, 

 and we think we have every evidence that can possibly be 

 adduced in support of our opinion. In detailing the cha- 

 racters of our Avimla, we shall therefore endeavour to 

 point out and elucidate those particulars in which we 

 think Lamarck has erred in separating them. Both these 

 Lamarckian genera are placed by Linneans in Mytilus, 

 but it would now be almost needless to enumerate the 

 characters by which they are to be distinguished; the 

 general form and the inequality of the valves are alone 

 sufficiently discriminative marks. 



The general form of these shells is rather square, 

 with the superior angles rounded ; in the Lamarckian 

 Meleagrina? it is nearer to orbicular than in his Avicidse, 

 which are rather more transverse, but there is decidedly a 

 transition in form : the two valves are unequal^ in the 

 Aviculce rather more conspicuously so than in the Melea- 

 grince, but distinctly so in both, and particularly in the 

 pearly part within. The base or hinge line is transverse, 

 straight, its extremities sometimes very short, as in the 

 Meleagrince, but very variable in length, and sometimes 



