CRENELLA. 197 



less abruptly, at the commencement of the posterior compart- 

 ment, so as to render the latter sublobated in appearance. 

 The hinder dorsal edge sloping upwards in a nearly straight 

 and moderately ascending line unites itself with scarcely 

 any angulation to the posterior outline : this latter forms 

 an uninterrupted sweep to the commencement of the sublo- 

 bated portion ; its chief swell being nearly in the middle, 

 whence it so runs downwards as to give a slightly trunca- 

 ted look to the lower part of the hinder extremity. Both 

 extremities of the shell are rounded, though neither of 

 them symmetrically so. The umbones are not prominent ; 

 the beaks are nearly terminal, acute, and much inflected. 

 The interior is iridescent white, with its margin only cre- 

 nated at the extremities. 



A large individual now before us, measures seven tenths 

 of an inch in length, and five lines in breadth. 



" The animal," says Mr. Alder, in his Catalogue of the 

 Mollusca of Northumberland and Durham, " is white, and, 

 assisted by its long strap-shaped foot, can move about 

 pretty quickly ; but it generally prefers a stationary life, 

 and forms for itself a kind of nest or case, by stitching to- 

 gether the small sea-weeds and corallines with its byssal 

 threads ; here it remains attached by its byssus, awaiting 

 the food that may come within its reach. When viewed 

 in a living state there appear to be two syphons at the 

 longer end of the shell, but only the posterior of these has 

 the walls complete ; the other has its anterior side open, 

 formed by a fold in the cloak, as in the siphons of the 

 zoophagous gasteropods. The animal has consequently 

 only two pallial apertures. " 



Common all round our shores, and often very plentiful 

 among the roots of Laminarke and among corallines. It 

 ranges from low-water mark to thirty fathoms, at which 



