88 GASTROCILENID.E. 



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

 margin, and the 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 hasal 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 by 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 o])posite 

 valve ; muscular impressions obscure. Length, one inch ; height, 

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



Found adhering to marine olyects. They may almost always be 

 found among the roots of fuci, which are thrown up l)y storms, ad- 

 hering to stones, shells, &c. The best I ha\e ever obtained were 

 taken from a log drawn out of one of our tim])er docks, to which 

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

 the base. 



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



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

 given that is not lialjle to mislead. I think there can be little doubt 

 that the same shell exists on the Euro})ean shores, and that it has 

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

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

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

 Avhich Mr. Say applied to it. [Changed in the ])resent edition. 



In the first place it would come under the genus Byssomya, on 

 account of its being furnished with a byssus. 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 S. pholadis, Lam., the Mi/a 

 bi/ssifera, Fabr. These are found in places where their regular 

 growth is unobstructed. Other specimens, and especially adult 

 ones, seem not to vary from S. rug^osa. Nothing could apply bet- 

 ter to our shell than Turton's description of M/jtilus rug-osus, 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 ])orer like *S. rug-osa; indeed, there arc 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, 



