172 DESCRIPTION OF SHELLS. 
Pinna margaritacea. 
This seems to differ from Pinna affinis only in having more numerous and 
distinct radii. 
CONCHIFERA MONOMYARIA. 
Lima expansa. (Tab. III. fig. 34.) 
Spec. Cuar.—Nearly orbicular with one oblique straight side, compressed, 
radiated ; radii about forty, smooth, with concentric striz between them ; ear 
small, rectangular ; hinge-line short ; shell thin and tender. 
Much more orbicular and compressed than any other Lima I know, but much 
too oblique for a Pecten. It is extremely rare; Mr. Edwards has one nearly 
perfect valve, and Mr. Dixon a fragment. 
Pecten squamula. (Tab. III. fig. 29.) 
A rare shell; it is in Mr. Edwards’s Cabinet. 
Pecten reconditus (Tab. III. fig. 27) ; and P. plebeius, figs. 28 & 32. 
We are obliged to Mr. Nyst for having set us right about Brander’s shell, 
which is more oval than the shell given under that name in the ‘ Mineral Con- 
chology.’ Whether figs. 28 & 32 be rightly referred to P. plebeius of Lamarck, 
I am uncertain. There are so many intermediate forms and the surface varies 
so much in roughness with the state of preservation, that it is extremely difficult 
to separate this group of Pectens, with twenty to twenty-five compound rays, 
into well-defined species. Fig. 32 owes its irregular form to fracture; it has a 
coarser surface than usual. 
Pecten triginta-radiatus. (Tab. II. figs. 30 & 31.) 
Spec. CHar.—Margin orbicular, with a rectangular beak ; compressed, radiated 
and minutely squamose ; radii about thirty, simple, rounded, or with a slight 
furrow along the middle of each, covered with minute, close, perpendicular 
scales ; furrows equal to the radii, and similarly squamose ; ears large, radiated. 
A broader species than P. plebeius, and, though similar, well distinguished by 
the number and simplicity of its rays ; in some of the furrows between the rays 
