ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 833 



of the valve, while the diductors are not separated by a ridge, and there is a pair of 

 strong, outwardly curved divergent vascular sinuses starting from its front end and 

 enclosing the ovarian areas. In the brachial valve the two pairs of adductors are 

 strongly marked, but rather small ; and there are distinct vascular sinuses diverging oh 

 each side from them. Davidson* showed these features fairly well in his figs. 16, 17. 



Horizon. — Saugh Hill Group. 



Locality. — Woodland Point. 



Orthis (Plectorthis) rustica, var. nov. paucicostata. 



(Plate VI, figs. 9-12.) 



1883. Orthis rustica, Sowerby, Davidson (pars.), op. tit., Mon. Brit. Foss. Brach., vol. v, Silur. Suppl., 



p. 226. 

 1883. Orthis calligramma } Dalman, var. plicata, Sowerby, Davidson, ibid., p. 226. 



Some of the larger shells from Woodland Point and Cuddystone Glen, which have 

 the general characters of O. rustica, differ by the smaller number, greater simplicity, 

 and regularity of the ribs on the surface, which also show strongly on the interior, as 

 in the variety rigida. Thus there are only from 25-30 primary ribs on a shell with 

 a width of about 20-30 mm. ; these ribs are all rounded and simple and straight and 

 of equal or subequal size, while at about half their length there may be intercalated 

 a shorter, smaller rib, and there is an absence of the concentric rugosities generally 

 noticeable. The internal characters appear to be identical with 0. rustica, but the 

 cardinal angles are usually more pointed and the ribbing is more like that of 0. plicata, 

 Sowerby. This variety is difficult to separate completely from O. rustica and its 

 variety 0. rigida, as it occurs associated with them, and transitional forms are 

 observable. Davidson labelled those from Cuddystone Glen 0. rustica, and those 

 from Woodland Point O. calligramma, var. plicata. It seems best to regard them 

 as a separate variety of O. rustica, and they may be designated paucicostata. 



Horizons. — (l) Saugh Hill Group ; (2) Camregan Group. 



Localities. — (l) Woodland Point; (2) Cuddystone Glen. 



Orthis (Plectorthis) rustica, Sowerby, var. rigida, Davidson. 



(Plate VI, figs. 13-19.) 



1847. Orthis rigida, Davidson, Oeol. Journal, p. 64, pi. xiii, fi>s. 1-4. 



1868. Orthis rustica, var. rigida, Davidson, Mon. Brit. Foss. Brach., vol. iii, pt. vii, p. 238, pi. xxxiv, 



figs. 18, 19. 

 1883. Orthis rustica, var. rigida, Davidson, op. cit., vol. v, Silur. Suppl., pp. 178, 226. 



This form is very abundant at Woodland Point, and ranges in size from 10-20 



mm. in length. The pedicle-valve is conical to moderately convex and is always 



swollen about the beak, which is somewhat incurved ; the brachial valve is nearly 



flat or only weakly convex, with a faint median depression in the less flattened 



* Davidson, op. cit., iii, p. 238, pi. xxxiv, figs. 13-17. 



