PHODUCTELL^ OF THE HAMILTON GROUP. 159 



Specimens of this species, both from Iowa and New- York, have in some instances 

 shown a very minute truncation at the apex ; and in one or two individuals 

 there is a barely perceptible indication of an area, which extends on either side 

 half-way from the apex to the extremities. 



This species has usually been referred to P. murchisonianus of De Koninck 

 (which is also represented as having a narrow area, and belonging properly to the 

 Genus Strophalosia). In the comparison of numerous forms which have been 

 described as distinct species, and which he refers to this one. Prof. De Koninck 

 remarks as follows : 



" The greater number of these species have been established only upon imper- 

 " feet specimens, and require a rigorous revision by their authors. The P. mur- 

 " chisonianus to which we must refer, according to M. De Vehneuil, the Stropho- 

 " mena pustulosa of Mr. Hall, is easily distinguished from the preceding species* 

 " by its area,! by the less profound insertion of its ventral valve, by the less con- 

 " siderable number of its tubercles, and by its transverse wrinkles. It is not 

 " always so easy to separate it from the P. subaculeatus." 



The species described by me as Sfropftomena pustulosa has been alluded to 

 under, the preceding species, and appears to me to be the dorsal valve of a smaller 

 species of Phoductella, the P. truncate. I can scarcely suppose that such extreme 

 variations occur in the same species, as are shown in these two forms ; and if so, 

 we must confess that we have very feeble means of discriminating species among 

 this group of shells. In the eastern part of the State, where the P. truncata is 

 abundant, I have not seen the larger forms, though they occur in the same asso- 

 ciation on the shore of Lake Erie ; yet in that locality we have at least fifty of 

 the smaller forms where we find one of the larger, and but rarely an individual 

 which might be doubtfully regarded as in an intermediate stage of growth. 



Geological formations and localities. It will be observed that this species ranges 

 from the Comiforous limestone to the Oolitic limestone, at the base of the Bur- 

 lington limestone in Iowa. It occurs at Williamsville in the Corniferous limestone, 

 and in the same rock in Ohio and in Illinois. It is found in the limestone of the 

 Marcellus shale on the shore of Lake Erie above BuiTalo, and in the shales of the 

 Hamilton group at the same place ; at Pratt's falls in Onondaga county ; at Mos- 

 cow in Livingston county, and other places. It occurs in the same horizon at 



• Produetiu gerardi. 



t Among the illustrations of this species by Prof. De Koninck, pi. xvi, flg. 3/, shows a narrow area and 

 mtnote foramen; while flgs. 3 4 and 3 e, which are similar views of other specimens, do not show an area, 

 and have every external appearance of bejng true Peodcotcs. The Strophomena membranacea of Van- 

 vxsM, supposed by that author to be identical with Leptana membranacea of Phillips, is also cited by 

 Prof. D« Koninck as one of the synonymes under Produetut murchisonianus. The species referred to by 

 Vakuxem is illnstrated under Productella hirtuta of the Chemung group, and is certainly very distinct 

 froni either of those In the Hamilton group or Corniferous limestone. 



