ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 847 



possible to refer it to the genus Pentamerus or Strichlandinia. In some respects it 

 resembles Protorhyncha nasuta (M'Coy) (see p. 937), but the. brachial valve in that 

 species has a lower and less swollen beak. Davidson's specimen, on which he founded 

 Sir. shallockiensis, is much less well preserved than his figure would lead us to imagine, 

 and he was evidently not satisfied with his determination, for he wrote on the back 

 of the tablet, " This specimen puzzles me." 



Dimensions. — Length, 32 mm. ; width, 31 mm. 



Horizon. — Whitehouse Group. 



Locality. — Shalloch Mill. 



Subgenus heterorthis, Hall and Clarke. 

 Orthis (Heterorthis ?) confinis, Salter. 



(Plate VIII, figs. 26-29, ? 30, ? 32, ? 33.) 

 1848. Orthis confinis, Salter, Quart. Journ. Geol. Soc, vol. v, p. 15, pi. i, fig. 4. 



1870. Orthis confinis, Salter, Davidson, Mon. Brit. Foss. Brach., vol. iii, p. 266, pi. xxxvi, figs. 1-3 (non 4). 

 1883. Orthis confinis, ibid., vol. v, Silur. Suppl., p. 180. 



The types of this species, founded by Salter * in 1848 and described again in 



1851, t came from the "limestone of the Stinchar River" and from Bogang [Bugon]. 



M'Coy | and Davidson also described it, but the latter was mistaken in quoting it 



from the Llandovery of Mulloch Hill, for it is an essentially Llandeilo species, and 



his figures do not give such a correct idea of the ribbing as Salter's. It should be 



also remarked that the pedicle-valve is shallower than the brachial, is subconical, 



gently convex (most so in the umbonal region), has occasionally a weak keel near 



the beak, but tends to be flattened or slightly concave towards the front, especially 



in the young shells. The brachial valve is always more convex and deeper than the 



other valve, and has sometimes a weak shallow median sinus or depression towards 



the front. With regard to the muscle-scars, Davidson simply stated that they were 



small, though the interior of a pedicle-valve was figured, and several interiors from 



Minuntion in Mrs Gray's collection were referred by Davidson to this species. The 



muscle-scar in the pedicle-valve is about one-third the length of the valve, broadly 



subpentagonal, strongly forked behind, the ends of the diductors being free and 



pointed, and the adductor scar is short. The diductors are elongate and subparallel- 



The interior of the brachial valve is not well known, but the cardinal process in the 



specimen (fig. 33), which is doubtfully referred to it, is short, oval and elevated, the 



cardinal plate is thickened, the crura are short, stout and divergent, and the 



muscle-scars are indistinct. Frequently a submarginal internal thickening of the 



shell is present. The ribbing varies somewhat, as mentioned by all authors, but 



the fasciculation is rarely visible, and the ribs are mostly primaries, as Salter 



showed clearly in his original figure. The specimens from Craighead and Bogang 



have fewer and coarser ribs than those from other localities, and those from Bogang 



* Salter, Q.J.G.S., vol. v, 1848, p. 15, pi. i, fig. 4. t Ibid., vol. vii, 1851, p. 170, pi. viii, fig. 5. 



1 M'Coy, Syn. Brit. Pal. Foss, Woodw. Mus., p. 215. 



