350 INTRODUCTION TO CONCHOLOGY. 



The sliell of the genus Mesoclesma is ovate, transverse, 

 somewhat triangular, equivalve, rather compressed, thick, and 

 generally closed. The hinge has two lateral teeth in each 

 valve, between which is situated a deep spoon-shaped cavity 

 for the insertion of a hgament. The muscular points of 

 attachment are unequal, and the impression of the mantle 

 exhibits a sinus on the posterior side. 



Daudin instituted the genus Ungidina for the reception 

 of a remarkable variety of Mactracea, differing in a peculiar 

 degree from any of his brethren as regards the position of 

 the ligament. The shell is nearly orbicular, rounded, 

 equivalve, subequilateral, and closed at the sides. A short 

 bifid cardinal tooth is obvious in each valve, with the 

 addition of a small tooth in one of the valves only, but no 

 lateral teeth ; and just within the anterior margin appears 

 an oblong narrowly-divided pit or cavity, containing the 

 ligament, which is also divided, and so near the margin as 

 to be partially visible externally. The muscular impression 

 of the mantle is entire, and distinctly marked. 



The genus AmpMdesma includes a numerous and well- 

 defined group, somewhat variable as regards their shells; 

 for some are transverse, others orbicular, but readily dis- 

 tinguished by the peculiar position of a duplex ligament. 



