BIVALVES. MACTRA. 45 



DIVISION III. SUBCORDATE, OBSOLETELY RIBBED, STRIAT- 

 ED OR SMOOTH. 



FAMILY 1. Obsoletely ribbed. 

 Laevigatum Smooth. 



FAMILY 2. Lightly striated, approaching smooth. 

 Lineatum Streaked. Grcenlandicum Greenland. 



Serratum Egg-cockle. JEollcum Janus. 



*Rubrum Red. 



MACTRA. KNEADING-TROUGH. 



Animal a Tethys: Shell bivalve, unequal sided, equivalve ; 

 middle tooth of the hinge complicated, with a small hol- 

 low on each side; lateral ones remote, and inserted into 

 each other. 



THE genus Mactra has little to boast of, either in re- 

 gard to beauty or variety. The number of species amounts 

 only to forty-two, and in those no great difference of 

 colouring or form is observable. 



The Mactra is usually of a triangular form ; but, in some 

 instances, it is rather oblong. 



The surface of the exterior is generally smooth, or mi- 

 nutely striated ; some exceptions, however, may be found, 

 as the M. plicataria, and others, which exhibit a wrinkled 

 or ribbed appearance, similar to that observed in the differ- 

 ent species of the Cardium, but in a reverse direction ; the 

 latter being ridged or plaited longitudinally,- while, in the 

 Mactra, the elevations and their adjacent grooves are plac^ 

 ed transversely. 



The generality of Mactrae are of a delicate construction, 



