SAXICAVA. 81 



is habitually left exposed, and not merely protruded at 

 rare intervals. No portion of this appendage can he 

 withdrawn into the shell; and the same is often the 

 case also with S. rugosa. The structure of the shell 

 must he cellular, because in fossil specimens the surface 

 when abraded or worn appears under the microscope to 

 be studded with circular pits. As Spengler well re- 

 marked, the shell is not unlike My a truncata, especially 

 in the large opening at the posterior end. Clark pointed 

 out its close relation to Saxicava, and Woodward has 

 satisfactorily ascertained and shown its generic place. 



It is the Glycimeris arctica of Lamarck, Panopaea 

 Spengleri of Valenciennes, P. Bivorw of Philippi, and 

 P. Middendorffii of A. Adams. 



) **m 2. S. RUGo'sA*,(Linne.) f>l ri 



Mytilus rugosus, Linn. S. N. p. 1156. 8. rugosa, F. & H. i. p. 146, pi. vi. 

 f. 7, 8, and (animal) pi. F. f. 6. 



BODY varying in shape from oval to cylindrical, greyish- 

 white more or less tinged with yellow, sometimes brownish- 

 yellow or orange: mantle very thick, coarsely and deeply 

 wrinkled : tubes very extensile, enclosed in a brown membra- 

 nous sheath to within a short distance from their extremities, 

 where they separate and slightly diverge ; orifices often pinkish, 

 fringed with a double row of short whitish cirri with trun- 

 cated points ; each tube has from 16 to 20; those in the outer 

 row are much smaller than the inside ones; excretal valve 

 bell-shaped, widely open : gills very narrow : palps small : foot 

 finger-shaped, rather long, extremely flexible and muscular. 



SHELL oblong, usually somewhat inequivalve but especially 

 in its free and younger state, slightly compressed except 

 towards the beaks, frequently gaping in the front or on the 

 antico-ventral side, as well as at the posterior end, thick, 

 opaque and lustreless : sculpture, coarse, distant, and irregular 

 concentric wrinkles; the posterior side is marked in young 

 and free specimens by a double ridge, which is usually spinous 

 or imbricated, and diverges from the beak in each valve 



* Wrinkled. 



