38 The Ottawa Naturalist [June- July 



separated by flat interspaces which are generally broader than the 

 diameter of the pits. Between 15 and 20 rows occur in the space of 

 1 mm. The general outline of the valves is transversely elliptical, 

 somewhat similar to that of T. puncto striata. 



Horizon and locality: Lower Trenton limestone; Wolfe Island, 

 Ontario, Station 222. 



Order Protremata. 



Family Strophomenidae. 

 Genus Plectambonites Pander. 



PLECTAMBONITES PUNCTOSTRIATUS n. sp. 



Plate I, figs. 15-17. 



Shell of medium size, transversely semi-elliptical in outline, 

 cardinal angles slightly acute but not auriculate; adult shells varying 

 in width between 16 and 21 mm., in length between 9.5 and 12 mm. 

 Surface of both valves marked by fine, thread-like, radiating lirae, 

 4 to 6 of which occur in the space of 1 mm. ; every second or third lira 

 slightly more prominent than the intermediate, newly developed ones; 

 depressions between lirae occupied by rows of minute punctures which 

 give the whole surface a finely rugose aspect. Dimensions of four 

 typical specimens are: width, 14.1, 16.0, 18.3, and 20.8 mm.; length, 

 7.8, 9.6, 10.3, and 11.7 mm. 



Pedicle valve more convex than in P. curdsviUensis, with the 

 point of greatest convexity a little behind the mid-length of the shell; 

 beak and cardinal area as in the general P. sericeus type; lateral 

 margins rounding broadly into the convex anterior margin. A low, 

 narrow, mesial fold originates near the beak and broadens anteriorly; 

 in most individuals this is a fairly conspicuous feature of the shell, but 

 in a few it is scarcely perceptible. Cardinal margin crenulated by a 

 series of oblique wrinkles which in most specimens make their 

 appearance within 2 mm. of the beak and become progressively longer 

 toward the cardinal extremities; about 4 or 5 wrinkles occur in the 

 space of 3 mm. and the angle between them and the hinge line is 

 generally between 30 and 40; in some individuals the crenulations 

 are scarcely perceptible but they are rarely entirely lacking. Interior 

 of valve not known. 



Brachial valve not positively identified. 



The shells subsumed here are most nearly related to P. rugosus 

 (Meek), but the present species differs from that one most con- 

 spicuously in the presence of a mesial fold and sinus as well as in the 

 greater equality of its radiating lirae. 



Horizon and locality: Lower Trenton limestone; Wolfe Island, 

 Ontario, Stations 222 and 308. 



