BRACmOPODA. 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 Hinuella. 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 Mekista 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 MERlSTELLA.f 



* It is obvious that the Hret 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 Gokioccblia, Hall, which was suggested in 1861 for the Alrypa unlsulcata (Fourteenth Re- 

 port New York State Cabinet of Natural History, p. 101), is an exact synonym for PKHTAooifiA, and may, 

 therefore, be stricken from the list of genera. 



