62 pectinid^:. 



P. opercularis may be readily known from either of 

 the two foregoing species by its circular form, greater 

 size, and nearly equal ears. The P. Audouinii of Pay- 

 raudeau can hardly be considered a variety. Its sculpture 

 is that of the P. lineatus of Da Costa, and only differs 

 from that of ordinary specimens in the scales being 

 more regular and continued across the ribs. The P. 

 subrufus of Turton is merely the young state, with a 

 straighter outline. Many other specific names have 

 been invented by authors for still more trifling varieties. 



4. P. septemradia'tus*, Miiller. 



P. septemradiatus, Mull. Zool. Dan. Prodr. p. 248, no. 2992. P. Danicus, 

 F. & H. ii. p. 288, pi. Hi. f. 1-2, 7-10. 



Shell nearly circular and equilateral, except as in the last 

 species, thin, somewhat glossy : sculpture, mostly 7 rounded 

 but compressed ribs (the middle one being the largest), which 

 are much narrower than the interstices ; the surface is covered 

 with minute and close-set longitudinal strise, which are crossed 

 by equally fine but more remote transverse plates, so as occa- 

 sionally to form prickles at the points of intersection ; the sides 

 are marked with short and stronger strise, placed at right 

 angles to the longitudinal striae: colour reddish-brown, mot- 

 tled or streaked with white : margins as in P. opercularis ; 

 slope below the byssal sinus seldom, and never distinctly, ser- 

 rate : beaks prominent : ears nearly equal in the upper valve, 

 but not so in the lower valve, in which the right ear is the 

 larger ; all of them are sculptured by ribs radiating from the 

 beak, as well as by fine and numerous striaa, which run from 

 the sides or shoulders and diverge outwards ; there are also 

 some wavy striaa in the line of growth, crossing the auricular 

 ribs ; the ears of the lower valve project beyond and overlap 

 those of the upper valve, but much less than in any of the pre- 

 ceding species ; byssal notch slight : hinge-line straight : car- 

 tilage small : ligament very thin : hinge-plate rather broad, 

 microscopically striate across ; transverse rib slight and raised 

 a little on each side of the cartilage -pit : inside fluted or 



* Seven-rayed. 



