LUCINA. 53 



which form very obtuse angles a little before the middle, 

 whence they bend backwai'ds with a slight retusion, and a 

 trifling declination. In the largest example we have seen 

 the subcentral angles are replaced towards the lower edge 

 by somewhat flexuous striulae, which are arcuated in a 

 direction contrary to the concentric lines of growth. In 

 this shell, too, although the striulai extend to the hinder 

 extremity, they are interrupted in front upon the upper 

 dorsal area (which is somewhat flattened), by being broken 

 into several minute and extremely short zigzags, but ter- 

 minate, however, in their original subretuse downward 

 inclination. 



The ventral margin is excessively arcuated, and ascends 

 rather the more behind. The posterior side is the shorter, 

 and appears very high-shouldered, owing to the extremely 

 trifling declination of its but slightly convex dorsal edge ; 

 the upper corner is broadly subangulated. The front dorsal 

 edge is likewise but very moderately sloping ; it is short, 

 and forms a very wide angle with the rather straightish 

 upper part of the anterior edge. Both extremities are 

 rounded below. The umbones do not greatly project, but 

 the beaks are very acute, lean considerably forward, and 

 are preceded by a small but distinct oblong lunular de- 

 pression. "The ligament is not conspicuous externally, and 

 there is no dorsal depression around its site. The hinge of 

 the right valve is furnished with a single primary tooth, 

 that of the left with two ; there are, moreover, two lateral 

 laminae in both valves. The inner margin, when viewed 

 by the lens, appears most delicately crenulated ; the front 

 muscular scar is short and oblong. 



Montagu''s own specimen (still in our National Mu- 

 seum) is the largest we know of, measuring nearly half an 

 inch in breadth, and rather less in length. The only other 



