222 ORGANIC REMAINS. 



Mr Sowerby has enumerated eight species of plagiostoma ; 

 but they may perhaps be all reduced to two or three. If every little 

 difference in the shape or the marking of shells be admitted as the 

 foundation of a new species, we shall find one in almost every new 

 specimen. 



Fig. 3 of PI. VIII is a most beautiful specimen of Mr. Sowerby's 

 jylagiostoma rigida, from the oolite near Malton. One of the ears is 

 broken off, and indeed few specimens have both perfect. The shell is 

 handsomely marked with slender longitudinal ribs, somewhat zigzag, 

 or waved ; and these are crossed in various places with concentric 

 markings, or rings of growth, especially toward the base, where these 

 circular lines usually form a rich border, as is the case in this specimen. 

 As the original brownish or purplish colour of the shell partly remains, 

 and as the concentric lines are here of a lighter coleur, the beauty 

 of the border is the more striking; and even the zigzag disposal of 

 the ribs contributes to enhance it. 



It is observable, that though this is a left valve, it has the trian- 

 gular cleft at the beak very conspicuous, and Mr, Sowerby's figure 

 seems to have the cleft in the same valve.* In most specimens which 

 we have examined, the right valve displays the cleft ; or rather, we 

 may say, the principal cleft; for both valves contribute to form the 

 aperture at the beaks. The differences on this head do not appear 

 to constitute different species. The same remark applies to the size 

 of the shells, and still more to the concentric rings that traverse 

 the ribs, as these are scarcely alike in any two specimens. We may 

 even extend this observation, in some measure, to the general shape of 

 the, shell. Some specimens, like the one figured, resemble a cockle 

 or a pecten ; some have the oblong and wedge-like shape of a donax ; 



* Mineral Conchology, Tab. 113. Fig. 1. By some oversight. Tab. 113 in the engravings, 

 here referred to, corresponds with Tab. OXIV in the printed description ; and Tab. 114 corre- 

 sponds with Tab. CXIII. 



