LamelUbrdnchiate Sliells on the Hudson Eicer Group. 137 



REMARKS ON SOME LA3IELLIBRANCHI ATE SHELLS 

 OF THE HUDSON RIVER GROUP ^ WITH DESCRIP- 

 TIONS OF FOUR NEW SPECIES. 



By R. p. Whitfield. 



I lately received from Prof. J. Mickleboroiigli, of Cincinnati, Ohio, 

 several examples of Lamellibranchiate shells, collected from the softer 

 clay layers of the Hudson River Group, in the vicinity of Clarksville, 

 Clinton Co., Ohio, with a request that I would determine their specific 

 relations. 



On examination, I find some of them presenting features of con- 

 siderable interest, as showing the great degree of compression, or dis- 

 tortion, they have iindergoue, during the consolidation of the material 

 in which they were imbedded; and showing, what appears to me, proof 

 of the great amount of vertical compression, or shrinkage, which had 

 taken place in some of the softer la3'ers of the formation before they 

 became finally fixed or hardened. Sufficient, as the examples show, to 

 produce an imperfect slaty lamination through the substance of the 

 fossils. 



One of the examples referred to, is a large specimen of Cypricar- 

 dites Sterlingensis{?) M. & W., which in its pei-fect uncompressed con- 

 dition, must have had a thickness, measured through the body of the 

 valves, of at least one and one fourtli inches, which has been reduced 

 by compression to eleven sixteenths, or little more than half an inch. 

 The specimen has been imbedded in the shale with the plane of the 

 valves corresponding to the plane of stratification, thus bringing the 

 compression nearly or quite vertical to the longer axis of the shell, 

 thereb}^ reducing the thickness to nearl}^ one half its original dimen- 

 sions, without materially distorting or altering the general outline of 

 the shell; presenting in this condition, features, which would, under 

 ordinary circumstances, be considered as of specific importance. All 

 parts of the shell are proportionally reduced in one direction, while re- 

 taining their normal characters in the other. Other examples in the 

 collection sent, seem to have been imbedded in an opposite direction to 

 the one above mentioned; and these have been compressed in the 

 direction of the longer axis of the shell, thus reducing the length of 

 the shell veiy materially, while the thickness and height retain their 

 normal proportions. Lamellibranchiate shells are particularlj'- liable 

 to this kind of distortion, especially when imbedded in argillaceous or 

 shal}- rocks, and require the exercise of considerable judgment in 

 determining their specific relations, as they are liable to present A-ery 

 different characters, even when specifically identical. Where the 



