PRODUCTELLffE 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 Yerneuil, the Strophe- 
“ mena pustulosa of Mr. Hall, is easily distinguished from the preceding species* 
“ by its area,f 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 Strophomena pustulosa has been alluded to 
under the preceding species, and appears to me to be the dorsal valve of a smaller 
species of Productella, the P. truncata . 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 Brie ; 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 Corniferous 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 Buffalo, 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 
* Productus gerardi. 
f Among the illustrations of this species by Prof. De Koninck, pi. xvi, fig. 3/, shows a narrow area and 
minute foramen; while figs. 3 b and 3 e, which are similar views of other specimens, do not show an area, 
and have every external appearance of being true Productus. The Strophomena membranacea of Yan- 
uxem, supposed by that author to be identical with Leptcena membranacea of Phillips, is also cited by 
Prof. De Koninck as one of the synonymes under Produetus murchisonianus . The species referred to by 
Yanuxem is illustrated under Productella hirsuta of the Chemung group, and is certainly very distinct 
from either of those in the Hamilton group or Corniferous limestone. 
