210 Mr. Gray's Conchological Observations. 



front of the columellar edge of the aperture. In the True Bivalves^ 

 the length is from the anterior to the posterior margin, the depth 

 from the cardinal or dorsal to the basal or ventral edge, and the 

 breadth from the most convex part of the disk of one of the valves 

 to the sanie part of the other. In the Brachiopodous Bivalves the 

 length is taken from the hole, or gaping end of the shell, to the 

 opposite margin ; the breadth is from side to side of the edge of 

 the valves, at right angles with the length ; and the depth is from 

 the most convex part of one of the valves to the opposite part in 

 the other. And although this mode of measurement will appear, 

 when the shell alone is considered, exactly the reverse of that re- 

 commended for the other bivalves, yet with reference to the ani- 

 mal, it will be found perfectly to accord with the principles I 

 have previously attempted to explain. 



§ 4. On the Muscular Impressions of Bivalves. 

 ■ Lamarck has used the character offered by the impressions left by 

 the attachment of the adductor ligament and muscles of these shells, 

 to separate them into two groups, calling the one Dimyares, and 

 the other Monornjjares, as if there were a difference in the number 

 of the muscles ; but there are al.A'ays two, which only vary in po- 

 sition and size; thus in the latter, the posterior one is larger and 

 more in the centre of the shell than in the former; but if the 

 anterior part of the cavity be examined, the other will be found 

 of a larger or smaller size, according as the species approaches or 

 recedes from the type of these groups, which are very natural, 

 though like most otliers that have a significant name, they do not 

 agree with it in a literal signification. There are also two other 

 muscular impressions, close and sometimes confluent with the for- 

 mer, which support the body and foot of the animals; but there 

 are besides these, which have hitherto only been taken notice of 

 by conchologists, two others which give great assistance in the 

 natural arrangement of this diflicult class. I allude to the sub- 

 marginal impression, or that by which the beard or mantle is at- 

 tached to the shell, and the hinder impression of the adductor 

 muscle of the syphon, which I propose to call the si/phonal scar. 

 The presence or absence of this latter impression, at once points 



