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 Laminarise and among corallines. It 

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



