140 



Anterior extremity most projecting, at about half the height of the shell ; 

 the outline descending with a broad curve to the ventral margin ; 

 apparently a Httle concave at the upper three-fourths of the height just 

 below the beaks. Posterior extremity about two-thirds the height of the 

 shell, rounded or slightly angular at mid-height. 



The valves are moderately and somewhat uniformly convex, most tumid 

 a little above the mid-height and in the anterior two-thirds of the length. 

 From the umbones a narrow groove descends to about the middle of the 

 ventral margin. 



Surface of the anterior extremity with obscure rounded concentric 

 ridges, nearly two lines wide, narrower in the upper half and on the 

 umbones. On the sides and posterior half, in one specimen, a set of fine 

 striae four or five in one line are visible. 



The two specimens collected have the hinge line buried in the matrix, 

 and it cannot therefore, be determined, whether they possess the lunette 

 characteristic of the genus or not. 



Locality and Formation. — Arisaig, Nova Scotia. Upper Silurian. 

 Collector.— C Weston. 



Grammysia Acadica. (N. sp.) 



Plate 9, fig. 4, 4a. 



Description. — Shell oblong or obliquely ovate, strongly convex ; beak s 

 anterior j dorsal and ventral margins sub-paiallel, the dorsal often ele- 

 vated posteriorly. Anterior end often nearly vertically sub-truncate, (as 

 in fig. 4a.), sometimes more or less obliquely sloping backwards from the 

 beaks, and downwards to the ventral margin. The latter is generally 

 moderately convex, sometimes straightish in the middle, or with a slight 

 concavity in the anterior half. The most projecting point of the posterior 

 extremity is about the mid-height of the shell, where it is usually rounded 

 or obscurely angular ; above the middle, with a nearly straight, or gently 

 curved slope, forwards and upwards to the posterior end of the hinge line ; 

 below, rounded to the ventral margin. The hinge-line is straight or 

 sHghtly concave, and extends backwards to a point varying in position, 

 between the mid-length and the posterior fourth, usually about haif-way 

 between the two. The beaks are closely incurved, almost if not quite in 

 contact with each other. There is a small anterior lunette, often almost 

 obsolete ; a large one behind the beaks extends nearly the whole length 

 of the hinge-line. 



The lateral groove commences at the beaks, and runs backwards and 

 downwards, to the lower half of the margin of the posterior extremity. 



