AVICULA. 253 



body or main portion of the shell is (in the more typical 

 examples) very oblique and rather narrow ; the ventral 

 outline is strongly arcuated, forming one wide sweep from 

 the end of the lobe to the most produced extremity of 

 the body, which latter is decidedly but not immoderately, 

 shorter than the subtriangular wing, the sinus beneath 

 which is not particularly profound, and more frequently is 

 inferior to, than it exceeds, a right angle, but varies much 

 in depth and extent, according to the greater or lesser 

 elongation of the dorsal wing. Generally speaking, a 

 straight line drawn from the beaks to the most project- 

 ing part of the lower margin is equal or nearly so to 

 the extreme length of the wing edge. The trigonal lobe 

 is moderately large, and in the more convex valve as 

 broad as it is long ; the byssal passage prevents this 

 being the case in the lesser valve. The beaks are pro- 

 minent, and not usually adjacent to each other. The 

 lobes and the area between the body and the wing are 

 much compressed, and the dorsal or cardinal line is very 

 long. The sides of it are extremely unequal, the upper 

 edge of the wing being from four to six times the length 

 of the upper edge of the lobe; the greater dispropor- 

 tion more usually exists in the adult. There is a single 

 small blunt somewhat triangular tooth under the acute, 

 very oblique but inflected beak in one valve, and two 

 obsolete denticles in the other. The medial portion 

 only of the interior is covered with a thin stratum of 

 nacre, which is usually prismatic. 



The largest individual we have observed measures nearly 

 four inches in length. 



The little specimen (a single valve) delineated by Tur- 

 ton in both his " Dictionary ,-1 and " Dithyra," can only, 

 we imagine, be regarded as a variety of this shell, although 



