ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 919 



attributed these Craighead shells, came from Kildare, and is certainly distinct in 

 character, having mucronate wings and different ribbing ; and the Ardmillan 

 shells, which he also referred to this species, belong to the genus Leptella or 

 Plectambonites, and are described in this memoir as L. pseudoretrojlexa (see p. 874). 

 The flattened conical shape of the pedicle-valve, its obtusely pointed and lower 

 non-incurved beak, the flat (instead of concave) and more inclined hinge-area, and 

 the less regular and rather coarser riblets seem sufficient to distinguish this species 

 from the other species, H. Thomsoni, described below. 



Hemipronites Thomsoni, sp. no v. 

 (Plate XXI, figs. 27, 28.) 



1883. Orthis turgida, M'Coy, Davidson (pars), Mon- Brit. Foss. Brack., vol. v, Silur. Suppl., p. 187, 

 pi. xiv, fig. 18 (non figs. 17, 19, 20). 

 Non 1870. Orthis turgida, M'Coy, Davidson, ibid., vol. iii, pt. vii, p. 258, pi. xxxii, figs. 12-20. 

 Non 1852. Orthis turgida, M'Coy, Syn. Brit. Pal. Foss. Woodw. Mus., p. 229, pi. iH, fig. 20. 



Shell subcircular to subelliptical, strongly biconvex ; hinge-line rather less than 

 maximum width ; cardinal angles obtuse or rounded. Pedicle-valve convex, rounded, 

 not carinate, much longer than brachial valve, with high elevated pointed beak, slightly 

 incurved, and with cardinal slopes meeting at 80°-100° at beak ; hinge-area large, high, 

 triangular, lying nearly in plane of valve or slightly sloping back, concave, with 

 triangular open delthyrium. Brachial valve transversely elliptical, with traces of a 

 broad median depression generally present ; beak small, inconspicuous, obtuse ; hinge- 

 area narrow, steeply inclined. Surface of valves covered with very numerous closely 

 placed regular equal rounded low riblets dividing usually close to margin, and number- 

 ing about 150 on edge of valves, gently curved back on lateral portions of shell, and 

 all crossed by fine concentric striae, with 2-3 lamellose growth-ridges near margins. 

 Interior unknown. 



Dimensions. — 



Pedicle-valve. Brachial valve. 



(1) (2) (1) (2) 



Length .... 15'4 16"0 mm. 11*8 13'6 mm. 



Width . . . 16*0 18"0 mm. lG'O 18'0 mm. 



Horizon. — Stinchar Limestone Group. 



Locality. — Craighead. 



Remarks. — This shell does not seem identical with M'Coy's Orthis turgida * from 

 the Llandeilo of Llandeilo ; the ribbing is different, and M'Coy's type, which is a 

 brachial valve, is more convex and has a more marked rounded sinus. M'Coy's and 

 Davidson's figures are restorations, and inaccurate. The other specimens referred 

 by these authors to the same species are certainly also distinct from these Craig- 

 head shells, which much resemble H. infiexa, Pander, t in shape, ribbing, high 



* M'Coy, Syn. Brit. Palseoz. Foss. Woodw. Mus., 1852, p. 229, pi. iH, fig. 20 (? 21-24). 



+ Von Pahlen, Mem. Acad. ImpCr. Sc. Si Petersbourg, ser. vii, vol. xxiv, 1877, p. 28, pi. iii, figs. 1-5. 



