133 



extremity. Beaks situated at about the anterior fourth, closely incurved. 

 Anterior extremity about half the height of the shell, uniformly rounded 

 below, apparently narrowly rounded or sub-angular above. Posterior 

 margin obliquely sloping, from the end of the hinge line downwards to 

 about the mid-height of the shell, below this sub-truncate or broadly 

 rounded. From the umbones, an obtusely rounded angulation extends to 

 the lower posterior angle ; above this the shell has a flat, gently convex 

 or gently concave slope to the hinge-line and posterior margin. Below 

 and in front of the angulation the sides of the shell are gently convex, 

 or partly concave and partly convex. 



Surface with strong concentric ridges, four to six in two lines, at the 

 anterior end. On the sides and at the posterior extremity the ridges 

 are finer but accompanied by more or less strongly impressed concentric 

 grooves or interruptions of growth. 



Length of a large specimen, twenty-eight lines ; height of the same at 

 the umbones, twelve lines. 



This specimen is closely allied to 31. Pkitypliglla (Salter), and 

 should, perhaps, be regarded as merely a geographical variety thereof. 

 According to the figures in the (Mem. Geol. Surv. G, B. vol. 2, pi. 20) 

 the lower posterior angle of the English fossil is more pointed and the 

 ventral margin more widely sinuated. The anterior extremity is corru- 

 gated as in our species, but the sides and posterior extremity are smoother. 

 McCoy says : " Surface nearly smooth, with numerous small concentric 

 wrinkles of growth towards the margin." Pal. Foss, p. 268 il/. compJanata 

 (Sowerby,) is another allied form, and occurs in the same geological 

 horizon. It seems to be more prominently arched on the dorsal line ; 

 the anterior extremity is larger and the surface smoother. 



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



MODIOLOPSIS RUDA. (N. Sp.) 

 Plate 8, fig. 6. 



Description. — Shell transverse, length aboat twice the height ; beaks 

 nearly terminal ; dorsal margin straight, and gradually elevated from the 

 beaks backwards to a point a little behind the mid-length ; posterior margin 

 obhquely truncated, gently convex, nearly straight, forming with the dor- 

 sal margin an obtuse angle of about 135 ^ ; the lower ventral angle nar- 

 rowly rounded ; ventral margin nearly straight, sometimes gently concave 

 for about one half the length in the middle,*curving upwards at each end. 

 The anterior extremity is sma^l, half the height of the shell, projecting 



