192 OORBULIDiE. 



ceeded by any marked concavity. By this, and the ab- 

 sence of a Hgament, its external aspect is most readily 

 distinguishable from Hiatella arctica, to whose general 

 features it bears no slight resemblance. 



The interior is of an uniform white. The hinge consists, 

 in the left valve, of an erect, subtrigonal, laminar primary 

 tooth, which forms an obtuse angle at its apex, being sub- 

 truncated in front, and produced behind. It lies almost 

 entirely on the posterior side, and is subdivided by the 

 slightly more concave anterior portion, forming a shallow 

 cartilage pit : this division is likewise indicated by the 

 flexure of the upper margin, which, more arcuated in front, 

 becomes nearly straight posteriorly. A corresponding 

 deeply-seated tooth-receptacle is present in the right valve, 

 with, according to Turton, a denticle in front of it, which 

 latter, however, we have not discerned. 



The animal of this curious shell has been observed and 

 examined by Mr. Clark, from whose manuscripts we ex- 

 tract the following account of it, dated, " Exmouth, Au- 

 gust 7, 1836 : animal elongated, compressed, pale yellow- 

 ish-white ; mantle closed, except a passage anteriorly for 

 a small, narrow, subcylindrical foot of a bluish milky trans- 

 parent colour, having at its root a byssal groove, from 

 whence a few rather coarse filaments issue, by which the 

 animal is often attached ; posteriorly, the mantle forms the 

 anal and branchial tubes, which are very short and not in 

 the least divided ; both are at their margins furnished with 

 about eight or ten rather rough white cilia ; from the anal 

 tube there is frequently protruded a blue, milk-white, trans- 

 parent, tubular membrane or valve, which is more than twice 

 the length of the tube ; it is then suddenly retracted and 

 again protruded. The branchiaj and labia, on account of 

 the minute size of the shell, could not be observed." 



