RHYNCHONELLID. 187 
RuyYNCHONELLA Pentuanpica, Haswell. Pl. XXII, figs. 9 to 19. 
RuYNCHONELLA, sp., Saléer. Memoirs of the Geological Survey of Great Britain, 
Scotland (32), p. 138, pl. ii, figs. 7, 7a, 1861. 
— PENTLANDICUS, Haswell. On the Silurian Formation of the Pentland 
Hills, p. 31, pl. iii, figs. 9, 10, 1865. 
— = Dav. Traus. Geol. Soc. Glasgow, pl. i, figs. 22—27, 1868. 
Spec. Char. Shell small, oblong oval, ovate ; sometimes almost circular or transversely 
oval, rounded or truncated, and slightly mdented in front; broadest across the middle. 
Ventral valve convex, a little deeper than the opposite one, and longitudinally divided 
along the middle by an angular groove or depression, which, commencing close to the 
extremity of the beak, extends to the front, dividing the valve into two equal lobes; beak 
small, incurved. Dorsal valve moderately convex, with a slight flatness or longitudinal 
depression along the middle of the umbone. Surface of valves closely covered with 
numerous fine radiating stric or small ribs, which increase in number as they near the 
margins by the means of many interpolated striz. In the interior of the ventral valve a 
deep trilobed muscular scar is visible in the posterior portion of the valve, while in the 
dorsal valve a short small mesial ridge separates each pair of the adductor quadruple 
muscular impressions. ‘I'wo specimens measured— 
Length 6, width 5, depth 3 lines. 
» D, 2» 53, » 35 >» 
Ods. This small species is exceedingly variable on account of the modifications im 
shape it assumes, being either elongated or transversely oval, while both extremes are 
connected by intermediate shapes, the elongated oval being, however, the most common 
form it presents. It occurs in great abundance under the condition of external and 
ternal casts, no specimen with the shell preserved having been hitherto discovered. In 
1861 Mr. Salter recognised it as a distinct and probably new species; but though 
figuring it, he refrained from applying to it a distinctive specific denomination, ‘from its 
being too imperfect to identify with published forms.” It was subsequently named 
Rh. Pentlandicus by Mr. G. Haswell, who gave a description and figures of two of its 
principal modifications in shape. In Pl. XXII will be found a series of figures illustrating 
several of the variations in form assumed by this species, as well as carefully enlarged 
illustrations of its internal characters, selected from a very extensive series of specimens 
collected by Messrs. Haswell, Brown, and Henderson; Mr. Salter’s two figures have been 
likewise reproduced. By means of softened gutta-percha we have reproduced the per- 
fect shell (fig. 14), while fig. 19 is taken from the specimen described by Mr. Haswell as a 
distinct variety. 
Position and Locality. Rh. Pentlandica occurs by thousands in the Ludlow Shales of 
