ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 855 



Orthis (Dalmanetta) testudinaria, Dalman, var. nov. shallochensis. 



(Plate X, figs. 7-11.) * 



1883. Orthis testudinaria, Dalman, Davidson (pars), Mon. Brit. Foss Brack., vol. v, Silur. Suppl., 

 p. 226. 



Shell transversely semielliptical to subquadrate or subcircular ; unequally 

 biconvex to plano-convex ; hinge-line less than width of shell ; cardinal angles sub- 

 rectangular or obtuse. Pedicle-valve gently convex, subcarinate, and swollen near 

 beak ; beak moderate, sharp, rising a little above hinge-line, slightly incurved ; 

 hinge-area small, triangular, steeply inclined to plane of valve. Brachial valve very 

 slightly convex, flattened, divided by broad shallow sinus widening anteriorly ; beak 

 small, inconspicuous, not elevated ; hinge-area narrower and smaller than that of 

 pedicle-valve. Eibs subangular, numerous, curving gently outwards on each side, 

 unequal in size, the primaries being stronger; increasing in number to SO or 100 on 

 margin by rather irregular intercalation of a rather fine rib in each interspace at one- 

 third (and frequently another at one-half to three-fourths) their length, but rarely by 

 division of the primaries into two unequal ribs ; interspaces subangular, equal in size 

 to the ribs ; both crossed by fine concentric strise. Interior of pedicle-valve with 

 short broad feebly impressed diductor scar, not forked, obtusely pointed in front. 

 Interior of brachial valve with two fairly defined pairs of subcircular adductor 

 scars, separated by broad low median ridge ; crura short, stout, triangular ; cardinal 

 process small. 



Dimensions. — 



(l) 

 Length . . . .13 



Width . . . .18 



Horizon. — Whitehouse Group. 



Localities. — Shalloch Mill, Whitehouse Bay. 



Remarks. — Some of these shells were labelled by Davidson 0. testudinaria, 

 others 0. elegantula, and others 0. vespertilio. They cannot, however, be separated, 

 and the large number of specimens now available from Shalloch Mill prove that they 

 all must be included in one species. The ribs in all of them increase as a rule in 

 number by one or more intercalations (not by division, except very rarely, and then 

 near the lateral angles), but otherwise they could scarcely be separated externally 

 from 0. testudinaria as generally understood. Indeed, Schuchert * puts O. 

 emacerata, Hall, | and 0. multisecta, Meek, J which our Shalloch Mill form much 

 resembles, as varieties of 0. testudinaria. The muscle-scar in the pedicle-valve, so far 

 as it is known, does not agree with O. testudinaria as figured by Davidson,§ and thus 

 in addition to the ribbing there seems sufficient reason for separating it as a variety. 



* Schuchert, Bull. 87, U.S. Geol. Surv., 1897, p. 205. 



t Foerste, Bull. Denison Univ., vol. xiv, p. 21, pi. iv, fig. 1 ; ibid., vol. xvii, p. 128, pi. viii, fig. 3. 



\ Foerste, ibid., vol. xiv, p. 217. § Davidson, op. cit., iii, pi. xxviii, figs. 20, 21. 



(2) 



(3) 



17 



12 mm 



22 



14 mm 



