THE FAUNA OF THE 8T. JOHN GllOUP. 35 



uucleus by about a dozen straight radiating striiB ; these stride extend only one third from 

 the nucleus, but a few near the sides of the slope may, in some examples, be traced to the 

 margin of the operculum. The margin of the operculum is augulated at the junction of 

 the ventral and lateral slopes. The whole surface of the operculum is marked by numer- 

 ous, fine, concentric striœ,'with a few stronger lines of growth at intervals. 



The ventral surfiice of the shell of this species is marked by very fine, close-set, trans- 

 verse striae, varied by strong lines of growth at short intervals ; in some examples raised 

 transverse bands are found at intervals corresponding in distance to the diaphragms of 

 the lateral skeleton. The inner side of the dorsal surface is smooth and silky looking, with 

 faint undulations of growth, but under the lens is seen to have very delicate and rather 

 distant, longitudinal, and also similar, arched, transverse striœ. 



Among the Bohemian species we seem to have in H. ma.cimus, Barr., (PI. 12, Figs. 22-2Y 

 Syst. Sil. Bohém., Vol. III.) on a large scale, a type similar to this species ; the compressed 

 example figured presents a similar lateral furrow, but the operculum is not notched on the 

 margin. The operculum found with our species may be compared with one of the " Second 

 Fauna." from Bohemia, figured by M. Barrande (Plate 13, Fig. C, Syst. Silur. Bohém., 

 Vol. III.) This operculum and those of A and B of the same plate have the peculiar lateral 

 notch found in the opercula associated with the shells of Diplotheca. 



Length, about 25 mm. WidtJi, about 9 mm. Rate of tapering, 1 in 2?; mm. 



Locality s,\\à Harizon. In the fine dark shales of Div. l.d, at Porter's Brook, St. Martin's 

 Common. 



Diplotheca Acadica, var. a. sertcea. (Plate VI. Figs. 1, la and b.) 



Diflers from the type in the following particulars : — The shell is larger and more slen- 

 der, and is attenuated toward the apex ; the annulations or diaphragms at the apex are 

 closely set. 



The operculum found with this variety is longer than broad ; the notch at the side is 

 well marked, but the stri;e on the dorsal slope are short and faint ; the nucleirs is four 

 fifths of the diameter of the opercuhim from the front (dorsal) side. 



Length of shell, 38 mm. Breadth, 13 mm. Rate of tapering, 1 in 2i mm. 



Locality and Horizon, same as the type. 



Var. fi. OBTUSA. (Plate VI. Fig. 8.) 



Differs from the type in the folloAving particulars: — It is shorter and more obtuse 

 toward the apex ; there is a distinctly defined median ridge on the ventral side (like that 

 of D. caudata.) One or more distinctly marked septa at the bottom of the body cavity. 

 Lines of growth more distinct both on the dorsal and ventral side than on the type. 



Operculum iruknown. 



Length, 30 mm. Width, 12 mm. Rate of tapering, 1 in 2 mm. 



Locality and Horizon. In the fine grey shales of Div. l.c, at Hanford Brook, St. Mar- 

 tin's. Not rare. 



Var. y- CRASSA. (Plate VI. Fig. 9.) 

 Differs from the type in having the shell shorter and more veutricose at the body 



