DITHYROCARIS GLABRA. 137 



The dorsal junction of the valves was furnished with a flanged crest or ridge 

 (PL XXV, figs. 1, 2), such as occurs in B. granulata and other forms; and 

 though the valves appear to have been sufficiently convex (PL XIX, fig. 2) to 

 have formed a bivalve cai'apace, there is no direct evidence of this having been the 

 case. In its congener D. granulata, with which form it has much in common, the 

 moieties are rather convex, and yet the carapace seems to have been clypeiform 

 or Apus-like (PL XX, fig. 3). 



PL XVIII, figs. 1 a, h. Mus. Geol. Surv. ScotL, F ff, 4368, tablet 23, No. 11. 



Size. — Length of valve, imperfect, 53 mm. ; breadth of valve 25 mm. 



Characters. — A flattened subelliptical left valve, imperfect by the loss of its 

 posterior border and spine. Ventral margin elliptically curved ; dorsal edge 

 straight and simple. It has no mesolateral ridge, but a slight longitudinal undu- 

 lation is formed by the compressed convexity. The apparent shading in Fig. 1 a 

 is due to the thin shell being darkened by the black shale of the matrix, and it is 

 emphasised at its border by a crack showing the black matrix, especially at an 

 oval spot in the postero-dorsal region. 



There are slight inequalities of the surface anteriorly, some of which are 

 due to the presence of the gastric apparatus; there is an obscure nuchal ridge. 

 The ventral border bears a fringe of closely-set, small, raised striae, or com- 

 pressed spinules, pointing outwards and backwards ; they die out anteriorly, to 

 be replaced by small marginal prickles analogous to the ends of the strisB. 



The anterior process near the middle line or axis of the valve consists of a 

 group of small spines (about six) rising from amongst an obscure lattice-work of 

 angular scales, which die out ventrally in oblique striae, and are replaced dorsally 

 by scattered tubercles on the antero-dorsal sinuous curve of the valve as far as 

 the small nuchal ridge of stronger tubercles. They are continued still further 

 along the dorsal region as minute tubercles (especially in fig. 2), spreading out 

 near the margin as far as the postero-dorsal notch. The mai'gin then curves 

 down boldly to the strong posterior spine, the lower edge of which is continuous 

 with the ciirve of the ventral border. 



The specimen shown by fig. 1 a has been described and figui^ed in the ' Geol. 

 Mag.,' dec. 2, vol. i (1874), p. 108, pi. v, fig. 4. 



In fig. 1 h the surface, when the outer pitted film of shell is removed, shows a 

 very minute reticulation with porous meshes. 



From black shale, non-calcareous. Cement-stone group, Lower Limestone 

 series ; Glebe, Bast Kilbride. Collected by Mr. A. Paton. 



PL XVIII, fig. 2. Mus. Geol. Surv. ScotL, F \{, 4078, tablet 23, No. 12. 

 Size. — Length of valve 55 mm., including both the anterior and the posterior 

 spines. Breadth of valve 22 mm. 



