852 DR F. R. C. REED ON THE 



valves also are rather different ; so that we may probably regard these Balclatcliie 

 shells as constituting a variety (PI. IX, figs. 23-27). 



Orthis {DalmanellaV) girvanensis, Davidson (emend.). 

 (Plate IX, figs. 28-30.) 

 1868. Orthis girvaniensis, Davidson, Mon. Brit. Foss. Brach., vol. iii, pt. vii, p. 216, pi. xxviii, fig. 10. 



Davidson's description of this species leaves little more to be said. It may be 

 remarked that the convexity of the shell varies, but usually the brachial is more 

 convex than the pedicle valve. The ribs are very regular, close, filiform, and rounded, 

 and they increase by unequal bifurcation at one-fourth to one-half their length, or 

 by intercalation at about one-fourth their length ; and they scarcely curve at all 

 backwards, even on the lateral and cardinal portions of the shell. 



Though the interior of the examples from Craighead, which is the typical locality, 

 is unknown, there are some internal casts of brachial valves from Ardmillan, probably 

 referable to this species, which show a thin straight laminar cardinal process in- 

 creasing slightly in thickness towards its base, where it is attached to a median 

 thickening of the shell extending forwards as a low broad ridge dying out anteriorly. 

 The posterior adductor scars are small and faint, and from them diverge anteriorly 

 two low narrow ridges extending forwards from half to three-fourths the length of 

 the shell. The crura are large, stout, and triangular. 



This species may especially be compared with 0. conferta, Lindstrom,* which it 

 much resembles in shape, convexity, and ribbing, but our Craighead specimens 

 possess no median depression in the brachial valve, though the supposed Ardmillan 

 examples show it. 



Horizons. — (l) Stinchar Limestone Group ; (2) Balclatcliie Group ? 



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



Orthis {DalmanellaV) Rankini, Davidson (sens, restr.). 

 (Plate IX, figs. 31-37 ; Plate X, figs. 1-3.) 



1883. Orthis Rankini, Davidson, Mon. Brit. Foss. Brach., vol. v, Silur. Sujipl., pp. 190, 226, pi. xi, 

 figs. 17-18, J 19. • 



The types of O. Rankini as described and figured by Davidson in his Supplement 

 are from Ardmillan Brae, but he intended the name to cover all those forms which 

 he had previously {op. cit., iii, pi. xxviii, figs. 13-24, p. 227) included under the 

 name O. testudinaria. With this view it is impossible to agree, and we must there- 

 fore limit the application of the name strictly to these Ardmillan shells. - Davidson's 

 diagnosis of the species is barely sufficient to differentiate it from other similar 

 forms ; but his figures to some extent make up for the inadequate description, 

 though they are not wholly satisfactory. His fig. 17 is the external impression of a 



* LindstrOm, Fragm. Silur., p. 26, pi. xiii, figs'. 1-3. 



