830 DR F. R. C. REED ON THE 



linear cardinal process and very stout short crural plates ; adductor scars rather 

 deeply sunk, separated by broad rounded ridges about one-third the length of valve. 



Ribs 12-24 in number, angular or subangular, simple, coarse, regular, straight, 

 decreasing in size towards the cardinal angles, separated by angular or subangular 

 interspaces equal in size to ribs, both ornamented with a few fine longitudinal lines 

 and closely set concentric striae. Shell substance pierced with a few large punctse 

 or tubules. Ribs show marginally inside valves. 



Dimensions. — (l) Craighead form, average length, 10-12 mm. ; (2) Balclatchie 

 form, average length, 12-15 mm. ; (3) Shalloch Mill form, average length, 10-14 mm. 



Horizons. — (l) Stinchar Limestone Group; (2) Balclatchie Group; (3) White- 

 house Group. 



Localities. — (l) Craighead, Minuntion, Tramitchell, Doularg, Bogang ; (2) Bal- 

 clatchie, Ardmillan ; (3) Shalloch Mill. 



Remarks. — This shell undoubtedly belongs to the group of 0. calligramma as 

 understood by Wysogorski,* but cannot be regarded as a typical example of this 

 species, though included in it by Davidson and most authors. It- differs in the 

 angularity, greater coarseness, and smaller number of the ribs, and in these respects 

 resembles Wysogorski's 0. Schmidti and 0. callactis, Dalm. The variety of 

 0. callactis figured by Lindstrom f differs in having the ribs less angular, but 

 otherwise is much like our Girvan form. The Craighead specimens have rather 

 fewer ribs and a lower hinge-area on the pedicle-valve and are usually smaller 

 than those from the Balclatchie Beds, but otherwise seem specifically inseparable. 

 Wysogorski's O. lyckholmiensis (op. cit.), as interpreted by Wiman,| may be corn- 

 pared as regards ribbing, but it has a lower beak to the pedicle-valve and the shell 

 is biconvex, besides occurring at a higher horizon. Gagel's § 0. calligramma may 

 be identical with our form. It appears advisable to separate the latter from the true 

 O. calligramma, and it may be distinguished as 0. Playfairi. It is probable that 

 Davidson's O. calligramma of his pi. xxxiv, figs. 1 and 3, from Moelydd, is 

 identical, but it is doubtful if the specimen from Craighead, fig. 4, belongs to the 

 same species, as the ribs seem rounded and more widely spaced. Dr Matley || has 

 recently discussed the synonymy of O. proava, Salter, which is closely allied to 

 our Girvan shell ; but I am not prepared to consider them identical, because the 

 Welsh shell has a more convex shape, a more deeply lobed brachial valve, and a less 

 distinct angularity of the ribs. The coarseness and especially the angularity of the 

 ribs recall certain examples of 0. actonise ; but the internal characters, the shape of 

 the shell and the less swollen and less incurved beak of the pedicle-valve distinguish 

 the present species. The internal characters are chiefly known from the specimens 



* Wysogorski, Zeitsch. deut. geol. Gesell., lii, 1900, p. 230, t. viii. 



t Lindstrom, Fragm. Silur., p. 26, pi. xiv, figs. 1-3. 



| Wiman, Arkivf. Zool., Bd. iii, No. 24, 1907, p. 8, t. ii, figs. 9-12. 



§ Gagel, Brach. camb. silur. Gesch., 1890, p. 33, t. ii, fig. 13. 



|| Matley, Appendix III, Summary of Progress of Geol. Surv.for 1911, 1912, p. 78. 



