36 MOLLUSCA FROM THE CRAG. 



sf/iiamoftiN, squamis crcbris, transversis, interradios divercativt striatis ; auriculis suh- 

 cBquah/jits ; vnlvd siiiisfrd convexiori. 



Shell suborbicular, slightly inequivalved, covered with 18 — 26 imbricated or 

 squamose rays, squamae numerous and close set, between the rays are nsible fine 

 divaricating striae ; auricles nearly equal ; the left valve, the more convex one. 



Diameter, 2| 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 sjiccics, 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 PajTaudeau, 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 

 squamae, and the spaces between the ribs arc 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 ra)'s, 

 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 

 s])ecies. In one variety of my Crag specimens, the rays are so strongly imbricated 

 with reflexed squamae, that in my Catalogue, it was considered a distinct species, 

 and intended to have been described under the name scabrotm (fig. 2, r) ; 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 shell, the 

 ribs are single, but the imbrications are continuous undulating over and between the 

 ribs. The var. Uneolata, I have seen only from the Red Crag, and that but rarely. 

 P. reconditas, 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 



