36 MOLLUSCA FROM THE CRAG. 
squamosis, squamis crebris, transversis, interradios divercatim striatis; auriculis  sub- 
a@qualibus ; valvd sinistrad convexiori. 
Shell suborbicular, slightly inequivalved, covered with 18—26 imbricated or 
squamose rays, squamz numerous and close set, between the rays are visible fine 
divaricating striz ; auricles nearly equal; the left valve, the more convex one. 
Diameter, 24 inches. 
Locality. Cor. Crag, Passim. 
Red Crag, Id. 
Mam. Crag, Bramerton and Thorpe. 
Recent, Britain, Finmark, and Mediterranean. 
This is one of the most abundant shells, in the Coralline, as well as 
in the Red Crag Formations, and is exceedingly variable as regards the ornament 
and arrangement of its exterior, which has caused it to be separated into 
many different species, so greatly, indeed, does it vary in this character, that 
scarcely any description can be given of its sculpture, but what some deviation 
may be observed, so as almost to induce an opinion, that such difference might 
be considered as a specific distinction. The most abundant variety is that which 
corresponds with the rough and imbricated shell, now found living in the 
Mediterranean, figured and described as a new species by Payraudeau, under the 
name P. Audouini. This shell may be found in almost every locality, in the 
Coralline as well as in the Red Crag. 
Both valves may be described as somewhat convex, though the upper or left valve 
is decidedly the more tumid of the two. Our shell is ornamented with more or less 
rounded rays, divided into threes, varying in number from 18 to 26, these 
are rather wider than the intermediate spaces, and are covered with rough imbricated 
squamze, and the spaces between the ribs are generally imbricated in the same way. 
In the young shell the tripartite form of arrangement is seldom to be seen, the rays 
then being single, and this continues sometimes till the shell has increased to more 
than an inch in diameter; and the division of the intermediate space into three rays, 
does not, in some specimens, show itself until even a greater magnitude, by which the 
young shell differs so materially in its ornament, as to have been made into new 
species. In one variety of my Crag specimens, the rays are so strongly imbricated 
with reflexed squame, that in my Catalogue, it was considered a distinct species, 
and intended to have been described under the name scabrotus (fig. 2, c) ; but the pos- 
session of more specimens and further examination, give reason to believe it to be only 
a modification of the above species: in this, which, is somewhat of a young sliell, the 
ribs are single, but the imbrications are continuous undulating over and between the 
ribs. The var. /ineolata, I have seen only from the Red Crag, and that but rarely. 
P. reconditus, Min. Conch., is I conceive, to be only that form sometimes met with in 
which the rays have preserved their unity until the specimen has attained a magnitude 
of an inch and a half in diameter, although in some specimens, they separate into threes 
