No. 109.] 



105 



Pentamerus galeatus. 



Pal. N.Y. Vol.iii, pl.46&47. 



This widely distributed species is characteristic of the compact limestones 

 at the base of the Lower Helderberg group, and is known to occur in the 

 same position as far south as Tennessee. 



It presents a great variety of form and surface markings. In New- York, 

 the extremely young specimens are always smooth, so far as I have ob- 

 served. In the various stages of growth, they present every possible degree 

 of development in the plications ; some individuals of nearly full size re- 

 maining smooth, while others are strongly plicated before reaching half the 

 full size. Some individuals present plications only on the mesial fold and 

 sinus. The plications are sometimes bifurcate, as shown in a single indivi- 

 dual on plate 46 and another on plate 47. 



The illustrations upon plate 46 show a few of the varieties of form and 

 marking to which this species is subject. 



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Pentamerus galeatus. 



