108 



of its being so plentiful, it is highly characteristic of the sand- 

 stone which I have named the Cyrena-bed. 



Some specimens which \ have succeeded in developing 

 from the sandstone agree , as above mentioned , with C. 

 Gravesi Desh. For comparison I have obtained specimens 

 from France, determined by Deshates himself. Like these the 

 Greenland specimens are comparatively high and greatly inflated, 

 the outline is almost circular. The umbo is large and promi- 

 nent. On some specimens at any rate, there seems at the 

 posterior region to be an intimation of a strongly rounded 

 keel. The surface is covered by numerous irregular growth 

 lines, that seem originally to have been fine, in most specimens 

 they are now rather coarse, which is "most likely due to the 

 bad state of preservation. The hinge seems to correspond 

 perfectly with the hinge of C. Gravesi Desh. Thus three 

 cardinal teeth are found, and the lateral teeth (especially the 

 posterior one) are long Hke those of the latter species. — The 

 only difference I have been able to find between the French 

 and Greenland specimens is that the latter seem to have a 

 somewhat thicker shell than the former. 



Some specimens differ somewhat from the above described 

 form, as they are higher and their keel is more prominent. Whether 

 they belong to the same or to several different species it is 

 impossible for me to decide on account of the incompleteness 

 of the material. The shells are all transformed into calcite and 

 therefore exceedingly brittle, so that the preparation of them 

 generally fails. As the sandstone contains calcium carbonate 

 as cement, it is also impossible to dissolve the shells, and by 

 means of the resulting moulds to study the construction of 

 the hinge. 



The species does not seem to attain the same size in 

 Greenland as in France, but a few of the Greenland specimens 

 reach the considerable length of about 40 mm., and about the 

 same height. 



