1ft ComCRYLlA—DITHYRA. I. 



very wide, and reaching nearly to the smaller end which terminates 

 in a rounded point, sloping suddenly at the anterior end into a short 

 rounded beak, giving that part an angular appearance : the anterior 

 part furnished with regular echinated striae separated by a fine de- 

 pressed line from the smaller part, which is marked with smooth 

 and rather distant striae : the hinge at the back covered with a long 

 flat process, reflecting backwards, and nearly in contact with the 

 back in each valve : on the inner or concave surface of the valves 

 is a short longitudinal rib, crossed by rough points, growing stronger 

 as it reaches the outer margin : this interior rib, which is always 

 visible in consequence of the very wide gape, will at all times dis- 

 tinguish the species. 



The accessorial valves are five in number : two shield-like ones 

 in front, which nearly close up the vast gape, and a narrow one down 

 the middle ; a heart-shaped one behind covering the hinge, and a 

 narrow one beneath it which connects the valves. 



It is probable that the presence or absence of the two shield-like 

 valves in front, occasioned Linne to form two species, and to cause 

 such uncertainty of identity among subsequent authors. 



Mrs. Grifiiths has procured a dead specimen of the Pholas crispata 

 from the rocks in the neighbourhood of Torquay, and ourselves have 

 taken the seven others; so that the whole of the eight British 

 species have been found in the highly productive shores of Torbay. 



