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MACTRA. 
Gen. Char. An equivalved subequilateral trans- 
verse bivalve,, gaping at the sides ; a ligament 
placed in a pit between the teeth of the hinge 
within the beaks ; two elongated lateral teeth 
in each valve. 
M ost of the shells of this genus are free from striee or 
other ornament ; their form approaches to transversely 
oval ; some are very flat, others are gibbose, and many 
have but a slight opening at the sides when the valves 
are closed : the lamellar lateral teeth of one valve are 
inserted into long grooves bounded on the inner side by 
plaits or lamellar teeth in the other : in several species 
both these teeth and the grooves are finely striated in a 
perpendicular direction. 
Lamarck has confined his genus Mactra to such spe- 
cies of the Linnean Genus as have prominent lateral 
teeth ; the remainder form the genera Crassatella and 
Lutraria. 
MACTRA arcuata. 
TAB. CLX. — Fig. 1 and 6. 
Spec. Char. Ovate, smooth, back and anterior 
margin arched; shell of an uniform thickness; 
posterior side smallest ; hinge narrow. 
XiENGTH equal to about four-fifths of the width : the lines 
of growth are strongly marked : it differs from Mactra 
solida in the narrowness of the hinge and the irregularity 
of the sides. The lateral teeth are striated. 
Mrs. Cobbold sent me these fine specimens from the 
Holywell Crag pits in 1813 : they had been compared 
with and received the name of Mactra solida of Linn. 
