450 DENTALIADiE. 



entails of the " Fauna Suecica" evidently, from its locality, 

 belongs to the present species. 



The tube is subarcuatetl and sub-cylindraceous, the cur- 

 vature at the broader end being only trifling, and the pos- 

 terior attenuation in the majority of examples by no means 

 rapid ; occasionally, however, this attenuation and arcu- 

 ation is less gradual in its progress. The surface, which is 

 nearly opaque and of a shining porcelain white, at times, 

 though rarely, ringed with duller or even fulvous lines of 

 growth, but never tinged with pink even at the narrower 

 extremity, is perfectly free throughout from all sculpture 

 whatsoever. The posterior termination has either a labial 

 projection which is rather broadly fissured dorsally {i. e. 

 upon the arched side of the tube) or if it have not experi- 

 enced that reparative process is then very tapering, and 

 has a short shelving notch-like dorsal fissure ; it is always 

 entire upon the ventral or incurved side of the shell. In 

 certain specimens the close approximation of the concentric 

 lines of growth produce a somewhat annulated appearance. 



Mature individuals of twenty- two lines in length only 

 measured a fifth of an inch at the broader end ; whilst the 

 diameter of the anterior extremity in a young and arcuated 

 example only an inch long, was all but the sixth of an 

 inch. 



Dr. Johnston has given a notice of the animal ; " the 

 form is like that of the shell, round and tapering gradually 

 to the posterior extremity; it is smooth, Avhitish,and closely 

 invested with a thin pellucid membrane, beneath which two 

 strong satiny ligamentous muscles are seen lying along the 

 ventral surface, adhering closely, and each of them divided 

 into a broad and a narrow slip * * *. The collar is very 

 thick and fleshy, and makes a complete circle, through 

 which the foot can be pushed at pleasure. The foot forms 



