862 DR F. R. C. REED ON THE 



being more transverse and less impressed. It more resembles some of Lindstrom's 

 forms* from Sweden which are doubtfully referred to this species, and differs from 

 the shell figured by WiMANf from the Leptsena Limestone by its more numerous 

 ribs. Davidson + records this species from Thraive Glen with a query. 



Subgenus harknessella, nov. 



Definition. — Shell subquadrate, unequally biconvex, with more or less pointed 

 cardinal angles, widest at hinge-line; anterior margin somewhat excavated. Pedicle- 

 valve with sharp median carination or angular fold, lateral angles somewhat flattened ; 

 beak small, pointed, only a little incurved. Brachial valve more or less strongly 

 bilobed by median sinus. Surface of shell ornamented with small bifurcating ov 

 fasciculate ribs. Interior of pedicle-valve with small teeth and elongated sub- 

 triangular or subpentagonal muscle-scar, slightly forked anteriorly and well marked. 

 Interior of brachial valve with large stout prominent oval cardinal process and very 

 stout thick triangular large subparallel crura, directed at about 75°-80° to hinge-line ; 

 cardinal plate much thickened and slightly produced in middle, with high front edge 

 excavated on each side. Shell-substance densely punctate. 



Type : Orthis vespertilio, Sowerby. § 



Sowerby's type of O. vespertilio came from the Bala Beds of Corton, near Clun- 

 bury, and consisted of internal casts of the two valves. From an examination of 

 specimens from this locality, as well as of many others from Wales and elsewhere 

 named by Salter and M'Coy as well as Davidson, the peculiarities of the species 

 seem so great as to necessitate its position in an independent group, no other species 

 appearing to be closely related to it, and the subgenus may be named Harknessella 

 after Robert Harkness. 



Orthis {Harknessella) vespertilio, Sowerby. 

 (Plate XI, figs. 3-7.) 



1839. Orthis vespertilio, Sowerby, in Murchison's Silurian Systevi, p. 640, pi. xx, fig. 11. 



1868. Orthis vespertilio, Sowerby, Davidson, Mon. Brit. Foss. Brach., vol. iii, pt. vii, p. 237, pi. xxx, 



figs. 11-21. 

 1883. Orthis vespertilio, Sowerby, Davidson (pars), ibid., vol. v, Silur. Suppl., p. 226. 



The occurrence of this species at Craighead was noted by Davidson, who stated 

 that it was very common there. The second example of the species which he figured 

 (op. cit., siqira, fig. 12) came from that locality. But I have not seen any undoubted 

 examples of it from the other localities which he mentions in his Supplement, i.e. 

 Shalloch Mill, Arclmillan Brae, and Doularg. 



The Craighead examples in Mrs Gray's collection are of rather large size, oue 

 measuring 36 mm. wide and 26 mm. long. The fasciculate subangular ribs are some- 



* Lindstrom, Fragm. Silur., p. 27, t. xiv, figs. 4-9. 



t Wiman, Arkivf. Zool, Bd. iii, No. 24 (1907), p. 8, t. ii, tigs. 8-86. 



\ Davidson, op. cit., Silur. »S'i/^p/., p. 226. § Sowerby, in Murchison's Silur. SysL, p. (>40, pi. xx, fig. 11. 



