181 



in the above description an elevated line, is, strictly speaking, the 

 line of junction with the basal surface which is a little more elevated 

 than the remaining surface of the volution, making a kind of step, 

 and not in reality a simple elevated line. In my observations on 

 this species, in the Journal above mentioned, I remarked that it 

 " very much resembles the clathratulus, Montagu, but the lip is 

 more robust and the basal portion of that part is more dilated." 

 PI. 27 ; lower left figure. 



Amphidesma. — Shell rounded or transversely somewhat oval, 

 subineqiiilateral : hinge with one or two cardinal teeth, and having 

 a lateral tooth each side, sometimes obsolete ; a slender oblique 

 fosset originating immediately under the beak and passing an- 

 teriorly : ligament divided into two parts, of which the true liga- 

 ment is exterior and very short, and the cartilage portion is interior, 

 affixed in the oblique fosset : muscular impressions two, remote, 

 not elongated : parallel line with a much dilated and profound 

 sinus. 



Ohs. — The characters of this genus are veiy distinct and easily 

 recognized from all those that have no part of the ligament inter- 

 nal. Lamarck first distinguished it by the name of jDonacilla, 

 which I think ought to be retained on the principle of priority, 

 and that of Amphidesma, that he afterwards applied to it, ought 

 to be rejected. He has associated it, in the same small group, 

 with JOngulina and Solemi/a, from both of which it may be known 

 by its much dilated and profound sinus of the palleal line. Other 

 genera of his family of Mactraceoe have divided ligaments, particu- 

 larly 3Iacfni, with which, in fact, Linne arranged the species; biit 

 their cardinal fosset is inore direct, dilated and deltoid, whilst that 

 of Amj)hidesma is nearly parallel with the edge, and fusiform. Lu- 

 traria is entirely destitute of lateral teeth, the valves gape and 

 the ligament is fixed in a deltoid cavity. Blainville unites Am- 

 phidesma with Lucina and with Fimbria, Megerle, or Corhis, Cuv. 

 But Corbis has lateral teeth, an exterior ligament, though deeply 

 seated 3 and the parallel line is entire, and the posterior muscular 

 impression is elongated. 



Amphidesma transversum. — Specific character. Trans- 

 versely short oval; hinge nearly central. 



Desc. — Shell transversely short oval, nearly equilateral, com- 

 pressed, a little gaping: anterior and posterior margins subequally 



