88 GASTROCILENIDyE. 



which two ridges or elevated lines run backwards, one near the 

 margin, and tlie other to the lower angle, giving the included sur- 

 face a lozenge shape. In some shells these lines are very distinct, 

 and they are armed with a series of elevated, arched scales or 

 spines ; the basal margin is usually contracted at the middle, and 

 slightly arched upwards ; surface coarsely marked with the lines of 

 growth, and irregularly undulated ; epidermis thin, dingy-yellow. 

 Ligament aided l>y the mantle, which adheres all along the back. 

 Teeth for the most part wanting ; when not wanting, a single rudi- 

 mentary tooth in one valve is received into a pit in the opposite 

 valve ; nuiscular imjircssions obscure. Length, one inch ; height, 

 three fifths of an inch ; breadth, two fifths of an inch. 



Found adhering to marine objects. They may almost always be 

 found among the roots of fuci, which are thrown ujj by storms, ad- 

 hering to stones, shells, &c. The best I have ever ol)tained were 

 taken from a log drawn out of one of our timljer docks, to which 

 they were adhering by a silken byssus issuing from the middle of 

 the Ijase. 



The foot of the animal is of a bright orange-color. 



This shell is a perfect Proteus, of which no descrijition can be 

 given that is not lialde to mislead. I think there can lie little doubt 

 that the same shell exists on the European shores, and that it has 

 been already described under at least one name. But, as 1 have not 

 the means of arriving at certainty on this point, I have chosen, until 

 better satisfied, and as it will introduce no new name, to retain that 

 which Mr, Say applied to it. [Changed in the present edition. 



In the first jjlace it would come under the genus Bijssomya^ on 

 account of its being furnished with a Ijyssus. But a majority of the 

 best modern conchologists regard this circumstance as of little im- 

 portance, and consequently reject the genus. Some specimens cor- 

 respond well with the description of K j/Iio/adis, Lam., the 3It/a 

 b//ssif('ra, Fahr. These are found in places where their regular 

 growth is unobstructed. Other specimens, and especially adult 

 ones, seem not to vary from S. 'n/i^osa. Nothing could apply bet- 

 ter to our shell than Turton's description of Mytilus rv^osns, in his 

 " Conchological Dictionary." But our shell is less likely to belong 

 to this than to *S'. pholadis, inasmuch as, l)esides the presence of a 

 byssus, our shell is not a borer like *S'. ritg-osa; indeed, there are no 

 rocks on our coast of a calcareous nature. 



Again, there are small specimens in which the two lines or ridges 

 along the posterior slope, armed with spines, are very conspicuous. 



