BRACHIOPODA. 
81 
portion of the upper surface of this plate bears a deep circular or crescentic 
concavity, most sharply defined on its anterior edge where it is bounded by the 
somewhat recurved vertical wall. On the lateral portions of the upper face of 
the anterior wall lie the elongate crural bases which are continued into short, 
straight crura, standing at an angle of about 45° to the plane of the horizontal 
face of the plate. The spiral cones are as in Meristella, their curvature con¬ 
forming to the peculiarly contracted interior cavity of the shell. Of the pre¬ 
cise nature of the loop we are still in doubt. The shells are not common and 
are rarely in a condition suitable for the successful development of their struc¬ 
ture. Figure 29, on Plate XLII, shows the extent of our knowledge in this 
direction, the specimen having a simple loop terminating in an undivided 
stem, not unlike that of Hindella. We are not, however, satisfied that the 
entire process is here retained, but enough is preserved to indicate that it 
may have been unlike that of Merista and Meristella. 
In the form of the shell itself there is an excellent ground for upholding 
the name Pentagonia, and as this is enforced by the character of the hinge- 
plate and probably, also, by that of the loop, the term may safely be adopted, 
though the genus has but a single known representative. For this two varietal 
names have been used ; (a) uniplicata., for the form with one pair of flanges on the 
brachial valve, (b) biplicata, for that in which these flanges are duplicate."^ The 
specimens from the Corniferous limestone of New York and the Falls of the 
Ohio, appear to be always of the uniplicate form, while in the Hamilton group, 
though the species is of much less frequent occurrence, both varieties are 
present. The characters of Pentagonia, so far as known, ally it most nearly 
with Meristella.! 
* It is obvious that the lii'st of these names, as it applies only to the typical form of the species, may be 
rejected ; the latter it will prove useful to retain. 
t The name Gonioccelia, Hall, which was sugg’ested in 1861 for the Atryim unisulcata (Fourteenth Re¬ 
port New York State Cabinet of Natural Histoi-y, p. 101), is an exact synonym for Pentagonia, and may, 
therefore, be stilcken from the list of genera. 
