140 



BRITISH SILURIAN BRACHIOPODA. 



(Clinton group) at Rochester ; near the Ridge Road in Ontario ; at Central's Mill, and 

 Sodus and Wolcott in Wayne County. In some beds it forms thin calcareous layers in 

 which the shells preserve a beautiful silvery lustre, as it does in the Tortworth district. 

 M. de Verneuil refers to the vast abundance of this shell both in Europe and in America, 

 Mr. Billings states it to occur in the Middle Silurian at South-west Point, the Jumpers, 

 and East Point in the Island of Anticosti. 



Atrypa? Scotica, M'Coy (sp.). PI. XIII, fig. 31. 



Ateypa hemisph^eica, Sow., var. Scotica, M'Coy. Salter, Quart. Journ. Geol. 



Soc, vol. vii, p. 178, pi. ix, fig. 12, 1851. 

 Hemithyeis — var. Scotica, M'Coy. Brit. Pal. Foss., p. 202, pi. i, h, 



fig. 10, 1852. 



Spec. Char. Shell small, transversely sub-orbicular, wider than long, or elongated 

 oval. Ventral valve moderately and evenly convex ; beak small, pointed, and incurved. 

 Dorsal valve very slightly convex, with a small longitudinal depression along the middle. 

 Sides and front rounded. Surface of each valve ornamented with from about twenty to 

 twenty-five small narrow ribs, usually bifurcated, the valves traversed by concentric lines 

 of growth. Two specimens measured — 



Length 5, width 6, depth 2 lines. 

 „ 6, „ 5i, „ 2 „ 



Obs. Prof. M'Coy himself suggests that " this may ultimately prove a distinct species, 

 in which case the varietal name may become specific." He points out that it is 

 distinguished from the type species {A. hemispJiarica) by the ribs being more numerous 

 (usually about twenty to twenty-four), considerably narrower, less prominent, and less 

 regular in size and distinctness, and often irregularly forked ; a character seldom or never 

 seen except at the inner edge of the flat valve in the typical specimens. The receiving 

 (ventral) valve is also slightly more carinate, and the entering (dorsal) valve not so perfectly 

 flattened, and with a faint trace of a wide undefined, mesial hollow." Having examined 

 a very great number of specimens of true A. hemispharica, as well as of A. Scotica, I have 

 preferred to retain them as distinct : in the first the ribs are fewer in number and simple, 

 while in all the specimens of the second that have come under my notice they were, as 

 the Professor says, much more numerous, narrower, and bifurcated. The question with 

 reference to the genus may, however, still remain undetermined. 



Position and Locality. It occurs in vast numbers in the greenish Lower Llandovery 

 sandstone of Mullock Quarry, above Dalquharran, near Girvan, Ayrshire. 



