890 DR F. R. C. REED ON THE 



in Mrs Gray's collection. Unfortunately his figure does not accurately or fully 

 represent the features which are present, so that a precise description and new 

 figures are requisite. We also know now the interior of the pedicle-valve, though 

 specimens of it are rare. In the case of the brachial valve the cardinal and umbonal 

 region- is not well preserved, but the adductor muscle-scars are well seen. These 

 muscle-scars are narrow, elongated, linguiform, compound, prominent, subparallel, 

 arching at first slightly outwards and then curving gently inwards to their anterior 

 ends, which nearly touch ; their edges are raised and sharp, and each scar is obscurely 

 divided down the centre into two subequal parts. They extend fully two-thirds 

 the length of the valve, and they rest on a thickened platform, most elevated 

 posteriorly. The narrow lanceolate area between the muscle-scars is depressed and 

 marked longitudinally by a median groove. The marginal portion of the valve 

 outside the muscular impressions is marked by fine short irregular radiating 

 vascular sinuses. 



The interior of the pedicle- valve shows a definite inner pair of narrow long 

 vascular trunks widely separated and somewhat arched outwards, extending nearly 

 the whole length of the valve, and having a broad area between them, finely striated 

 and pitted longitudinally ; outside this main pair of vascular trunks there is a group 

 of 5-6 subparallel short smaller vascular trunks running forwards and outwards. 

 No distinct diductors can be observed. Delicate radial lines of very minute papillae 

 cover the rest of the interior. 



The exterior of the shell appears to be practically smooth, the radial striae being 

 extremely fine and delicate and rarely preserved ; but concentric growth-ridges, 

 frequently sublamellose, are generally developed. 



This shell is particularly characteristic of the Craighead Limestone. In shape it 

 recalls Christiania tenuicincta (M'Coy), but the muscle-scars are much like those of 

 Plect. transversalis as figured by Hall and Clarke.* 



Horizons. — (l) Stinchar Limestone Group ; (2) Balclatchie Group. 



Localities. — (l) Craighead; (2) Balclatchie, Ardmillan. 



Genus Stropheodonta, Hall. 



The genus Stropheodonta was founded by Hall in 1852 with the Devonian 



species Leptaena deniissa, Conrad, as the type, and Hall and Clarke # state that 



the genus is " emphatically characteristic of the Devonian," and that it " makes its 



first appearance in the Clinton." Thus they do not recognise any representatives 



of it in Ordovician beds. Schuchert in Eastman Zittel's Text-book of Paleontology 



(2nd edit., 1913), p. 385, separates off as distinct subgenera the groups Leptostrophia, 



Douvillina, and Brachyprion mentioned by Hall and Clarke, but likewise only 



mentions them as occurring in Silurian or Devonian beds. The denticulation of the 



* Hall and Clarke, op. cit, Brach., i, pi. xv, fig. 36. 

 t Hall and Clarkk, op. cit., Brach., i, pp. 284-289. 



