ORDOVICIAN AND SILURIAN BRACIIIOPODA OF THE GIRVAN DISTRICT. 921 



pointed, small, slightly incurved, not swollen ; hinge-area large, high, triangular, 

 concave or flat, steeply inclined, with large triangular delthyrium. Brachial valve 

 shallow, very slightly convex or flattened, deeply bilobed by rounded median sinus, 

 with lateral portions of valve somewhat swollen ; beak small, inconspicuous ; hinge-area 

 triangular, much lower and narrower than in opposite valve, steeply inclined. Surface 

 of valves covered with 24-30 simple rounded equal closely placed low ribs, with those 

 in sinus of brachial valve rather smaller than rest ; fine concentric striation over all. 



Dimensions. — Length, 8-9 mm. ; width, 10—11 mm. 



Horizon. — Whitehouse Group. 



Locality. — Shalloch Mill. 



Remarks. — It is impossible to regard this shell as merely the young of Clitam- 

 bonites [Scenidium] shallochensis, Davidson, as might at first sight be suspected, 

 for the subcarinate contour of the pedicle-valve and the absence of the median sinus 

 upon it are distinctive. Further differences are mentioned in the description of that 

 species (see p. 917). Sc. Greenoughi more resembles Sc. Lewisi, var. asteroidea, from 

 Thraive Glen {q.v.), and Sc. Haiti, Safford,* from the Ordovician of the United States. 

 This species is named in honour of J. B. Greenough, one of the founders of the 

 Geological Society of London. 



Scenidium Lewisi, Davidson, var. nov. asteroidea. 

 (Plate XXII, figs. 1-3.) 



There are a few specimens of a Scenidium from the Starfish Bed in Mrs Gray's 

 collection which must be regarded as a variety of Sc. Lewisi; though not precisely 

 agreeing with any of the other varieties here described. The shell is less sharply 

 folded, and the pedicle-valve is less carinate and more rounded than in the variety 

 craigense, and agrees better with var. woodlandense. The ribs also are coarser and 

 larger than in the Craighead form. There are about 20 simple equal angular primary 

 ribs, with only 1 or 2 short marginal intercalated ones near the sinus in the brachial 

 valve, this regularity and simplicity of the ribbing being a noticeable feature ; and all 

 the ribs are crossed by strong imbricating striae. The internal casts, which are well 

 preserved, show a spondylium in both valves and in front of it a short stout low 

 median septum in the pedicle-valve about one-fourth (or less) its length, while the 

 septum in the brachial valve is long and thin, being about three-fourths the length 

 of the valve. In the brachial valve the muscular scars are as represented in 

 Davidson's figure of the interior of Sc. LewisiA This new variety does not appear 

 to be very closely allied to the variety Hughesi, Dav. \ from the Coniston Grits. 

 The largest specimen measures 8 '5 mm. in width. 



Horizon. — Drummuck Group (Starfish Bed). 



Locality. — Thraive Glen. 



* Hall and Clarke, op. cit., Brach., i, pi. viiA, figs 33-39. 

 t Davidson, op. cit., vol. iii, pi. xxvi, fig. 9a. 

 I Davidson, ibid., p. 254, pi. xxxviii, fig. 26. 



