886 DR F. R. C. REED ON THE 



radii, of which the median ones are usually most conspicuous ; interspaces with 

 fine striae. 



Dimensions. — Length, 4'5 mm. ; width, 9'0 mm. 



Horizon. — Drummuck Group (including Starfish Bed). 



Locality. — Thraive Glen. 



Remarks. — The general shape and ornamentation suggest that this shell is a 

 variety of PI. sericea, but the internal characters seem to be distinct from the 

 typical form. 



Plectambonites subcorrugatella, sp. nov. 



(Plate XV, figs. 33, 34.) 

 1883. Strophomena corrutjatella, Davidson (pars), Mon. Brit. Foss. Brach., vol. v, Silur. Suppl., p. 227. 



Shell transverse, semicircular, widest along hinge-line, twice as wide as long ; 

 concavo-convex ; cardinal angles acute, about 60°, not produced. Pedicle-valve 

 gently convex ; beak small, slightly incurved ; hinge-area narrow, steeply inclined, 

 with triangular delthyrium ; surface with 10-12 strong primary radii and 6-9 fine 

 striae in the interspaces, and occasionally a shorter secondary radius equal in strength 

 to the primaries ; short arched transverse corrugations in each interspace. Brachial 

 valve gently concave ; beak inconspicuous ; interior with trilobate cardinal process 

 and muscle-scars supported on raised solid platform, highest and abruptly ending at 

 anterior end ; surface of platform bearing pair of large subtriangular scars with raised 

 edges, slightly diverging, and composed of two pairs of contiguous adductors ; 

 weak concentric submarginal thickening internally. Exterior ornamented like 

 other valve. 



Dimensions. — Length, 6'5 mm. ; width, 14*0 mm. 



Horizon. — Whitehouse Group. 



Localities. — Shalloch Mill, Whitehouse Bay. 



Remarks. — Though this shell has the same ornamentation as Stroph. cormgatella, 

 Dav., and was thus labelled by Davidson, yet the internal characters prove it to be 

 referable to the genus Plectambonites. In shape and convexity it resembles PI. sericea. 

 It is a rare form, only five specimens having come under my observation. 



Plectambonites transversalis (Dalman). 

 (Plate XV, figs. 35, 36.) 



1828. Leptiena transversalis, Dalman, Vet. Akad. Handl., p. 109, pi. i, fig. 4. 



1870. Leptse.ua transversalis, Dalman, Davidson, Mon. Brit. Foss. Brach., vol. iii, pt. vii, p. 318, 



pi. xlviii, figs. 1-9. 

 1883. Leptxna transversalis, Davidson (pars), ibid., vol. v, Silur. Suppl., p. 227. 



The Girvan examples of this species seem to belong to five types. One is the 

 characteristic form of the species as found in the Wenlock rocks of England, but it 

 does not apparently possess the divergent diductor scars shown by Davidson (op. cit., 



