162 PALEONTOLOGY OF THE EUREKA DISTRICT. 



sial fold having a shallow median sinus; beak short, incurved. Ventral 

 valve most convex towards the beak, flattening out in the broad mesial sinus; 

 the latter is marked by a low rounded median plication. 



Surface of dorsal valve marked by a single low plication on each side 

 of the mesial fold that corresponds to a shallow sinus each side of the pli- 

 cation bordering the large mesial sinus of the ventral valve; a few fine con- 

 centric lines of growth are visible by the aid of a strong lens. 



Dimensions: height of largest specimen in the collection, 9 mm ; width, 

 12 mm ; convexity of the two valves united, 8 mm . 



On a shell having a height of 4 mm , a transverse diameter of 5 mm , and a 

 depth of 2.25 mm for the two valves, the mesial sinus and fold are shown only 

 by an undulation in the front margin, the valves are subeqtial in convexity 

 and the beak of the dorsal valve is scarcely elevated above that of the ven- 

 tral. With the increase in the size of the shell, the features of the adult 

 gradually appear until, in the largest specimens, there is very little variation 

 in form except a greater or less convexity as the shell is more or less trans- 

 verse. 



Of American species Pentamerus subgldbosus M. & H. (Geol. Illinois, vol. 

 iii, p. 429, plate xiii, figs. 5 a-c), approaches this species in size and form, but 

 the numerous plications and different convexity of the valves distinguish 

 them. Pentamerus Hplicatus Schnur, of the Middle Devonian of England 

 (Davidson, Mon. Brit. Foss. Brach., vol. iii, p. 73), may also be compared 

 with P. Lotis. 



It is with considerable hesitancy that this Upper Devonian shell is 

 referred to a new species, as it may possibly be a form descended from P. 

 comis, of the Lower Devonian of the Eureka District. No young shells of 

 the latter species have been found that we can compare with the earlier 

 stages of growth of P. Lotis, but in the larger form of the two species de- 

 cided differences appear, as may be seen by a comparison of the figures 

 representing them. 



Comparing P. Lotis with smooth specimens of P. comis, from the Upper 

 Devonian horizon at Rockford, Iowa, we find a decided resemblance between 

 them, but not sufficient to unite them as one species. 



Formation and locality. Upper Devonian, on the west side of Apple- 

 gate Canon, White Pine Mining District, Nevada. 



