ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 827 



between muscle-scars and edge of valve ; whole interior of valve densely covered 

 with minute pits arranged in concentric lines. Exterior of shell unknown. 



Dimensions. — Length, 9'5 mm; width (maximum), 10"5 mm. ; height (maximum), 

 4'0 mm. 



Horizon. — Balclatchie Group. •> 



Locality. — Ardmillan. 



Remarks. — Only one internal cast of the shell above described has come under 

 my notice. It appears to be related to Ps. depressa (Eichwald), as interpreted by 

 Von Huene.* The minute pits show of course as tiny granules in the internal cast. 



Order PROTREMATA. 



^Genus Orthis, Dalman. 



The classification of the numerous species of this genus has been attempted by 

 several palaeontologists with .more or less success, but the most comprehensive and 

 detailed system is that put forward by Messrs Hall and Clarke, t The probability 

 of further additions to the number of groups or subgenera equivalent in value to 

 those which they proposed was, however, recognised by these authors, and it must be 

 confessed that their scheme is not entirely comprehensive or sufficient when applied 

 to British species. Some new divisions of subgeneric rank have accordingly been 

 here introduced. It may be mentioned that Schuchert| has elevated several of 

 Hall and Clarke's groups to the rank of genera, and has also used several new 

 subgeneric terms. But in the present state of our knowledge it seems wiser to group 

 all the species in one genus Orthis, while recognising the fact that it contains many 

 more or less distinct subgeneric divisions. 



Subgenus orthis, sens, restr. 



Orthis calligramma, Dalman, var. nov. craigensis. 



(Plate V, figs. 7-9.) 



H868. Orthis calligramma, Dalman, var., Davidson, "Mow. Brit. Foss. Brack., vol. iii, pi. xxxv, fig. 6 

 (non cet.). 



A form of 0. calligramma occurs at Craighead in considerable abundance, but 

 it does not agree strictly with the usual Welsh and English shells referred to 

 this species. It closely resembles Wiman's type § from the Orthoceras Limestone, 

 except in the more conical shape of the pedicle-valve and smaller incurvature of the 

 beak. In this Craighead variety the shell is subquadrate to subcircular and of rather 

 large size, averaging 20-25 mm. in length ; the pedicle-valve is subconical and 



* Von Huene, oji. cit., p. 249, t. i, figs. 24-27 ; t. ii, figs. 1, 2. 

 t Hall and Clarke, Palseont. N.Y., vol. viii, Brach., i, 1892, pp. 186-229. 



I Schuchert, in Zittel's Text-book of Palaeontology, vol. i (translation, 2nd edit., 1913), pp. 381, 382. 

 S Wiman, Bull. Geol. Instit. Upsala, vol. viii, 1907, p. 103, pi. viii, figs. 28-30. 

 TRANS. ROY. SOC. EDIN., VOL. LI, PART IV (NO. 26). 120 



