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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 22, 



plicated species have an acute beak and a non-punctated structure. 

 Following out this simple observation, in connection with the other 

 characters before mentioned, we arrive at constant and definite cha- 

 racters for two subdivisions of this genus, entirely dependent on the 

 following general proposition : viz. that certain positions of the fora- 

 men with regard to the beak, its form, character and relation to 

 the deltidial area, always accompany the presence or absence of a 

 punctated structure. To the two divisions thus established, we have 

 applied the terms Epithyridce. and Hypothyridce, as suggested by 

 Professor Phillips. 



Typical forms of Terebratulae. 



Fig. 1. T. ovalis, Lam., showing the truncated form of beak and large deltidial area (Epithy- 

 RiD^) : 1 a. front view ; 1 b. side view. 



2. T. Menardi, Lam., the deltidial area not surrounding the foramen (EpiTHYRiDiE). 



3. T.plicatilis, Sow. (Hypothykid^). In consequence of the thickening of the margin of 



the foramen, the figure does not well illustrate this group, but the base of the foramen is 

 within the deltidial area :3 a. front view ; 3 b. side view. 



4. T. psittacea (HYPOTHYKiDiE) ; the plates of the deltidium are not joined in this species : 



4 a. front view ; 4 b. side view ; 4 c. view showing the beak more distinctly. 



In the first group, Epithyridae, the beak is always more or less 

 obliquely truncate, the foramen being almost entirely surrounded by 

 the substance of the dorsal valve and lying above the deltidial area*, 



* I have used the term deltidial area to express the small triangular space which 



