66 G. F. MATTHEW: ILLUSTRATIONS OP 



An example of this species has beeu fouud with valves expanded, but connected along 

 the hinge line. 



Length, 3 mm. Width of each valve, Bh mm. 



Horizon and Locality. In the sandy shales of Division l.lr, at Hauford Brook, St. 

 Martin's. Infrequent. 



Beyrichona tinea, n. sp. (Plate VI. Figs. 21, 21a and b.) 



Rudely subtriangular, with rounded angles at the hinge line and obtusely rounded 

 base. Depressed area extending less than half-way to the base of the valve ; distinct near 

 the hinge line, owing to the eleA^ation of the anterior and posterior ridges ; the rest of the 

 valve is tumid, especially on the posterior side. 



The anterior furrow is lunate, expanded at the hinge line, moderately impressed on 

 the front side, but obscurely on the posterior, and does not extend half-way to the base of 

 the valve. The posterior furrow is saucer-shaped, deepening on the posterior side, where 

 it is most distinct, and it extends about one third toward the base of the valve. 



The median ridge begins in front of the middle of the hinge line, and is very oblique^ 

 being directed toward the pit of the posterior furrow. The anterior ridge is subtriangular 

 and is highest in the middle, whence it slopes rapidly toward the anterior angle of the 

 valve. The posterior ridge is arched in at the upper extremity to a tuberculous prominence 

 at the hinge, and from this point extends along the hinge line as a low ridge parallel to the 

 median ridge. 



The marginal fold is distinct, narrow, and rounded. 



The surface is smooth, or minutely tuberculated, as appears from an examination with 

 the lens. Exfoliated examples of the valves appear to show the presence of two layers of 

 shell substance. 



Beyrichia Holli, Jones, of the Menevian, in the unusual width of the valves, and in 

 apparently having the anterior end wider than the posterior, approaches the genus 

 Beyrichona, but it is much smaller than either of the species above described. 



Length, 4 mm. Widtli, 4 mm. 



Horizon and Locality. In the sandy shales of Div. 1.6", at Hanford Brook, St. 

 Martin's. Frequent. 



PRIMITIA, Jones. 



This genus is represented by more than one species in the rocks of the St. John group, 

 and the author proposes to describe here a very tumid form, of which a few individuals 

 have been found. 



Primitia Acadica, u. sp. (Plate VI. Figs. 22, 22 a. and b.) 



Outline of the valves elliptical-oblong, with but very slight projection at the 

 extremities of the hinge line. The valves slope down in all directions to the margin, and 

 the form is so symmetrical that it is difficult to ascertain which is the upper and which 

 the lower side of the valve ; but the side on which an obscure marginal fold is found, is 

 supposed to be the lower side. It is also difficult to say which is the anterior and which 

 the posterior end of the valve ; one end has a low tubercle and a shallow pit, while the 



