ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT 817 



Genus Conotreta, Walcott. 



Conotreta conoidea, sp. nov. 



(Plate IV, figs. 1-5.) 



1883. Acrotreta Nicholsoni, Davidson (pars), Mon. Brit. Foss. Brack., vol. v, Silur. Suppl., p. 213, 



pi. xvi, figs. 21, 23 (non fig. 22). 

 1912. Acrotreta Nicholsoni, Walcott (pars), Camb. Brach. (Mod. U.S. Geol. Surv., vol. li, 1912), 



p. 696, pi. Ixxiii, figs, le, 1/, \g (non cet.). 



Shell very inequivalve. Pedicle-valve high, acutely conical, rounded ; height 

 usually greater than width or length ; width slightly greater than length ; apical 

 angle, 20°-30° ; apex sharply pointed, excentric, nearer posterior edge, perforated by 

 minute circular foramen ; posterior face below apex vertical, somewhat flattened, not 

 defined, with two very weak obscure furrows running down its centre from apex to 

 margin and faintly enclosing very narrow slightly depressed median triangular area 

 (often indistinct). Interior of pedicle-valve with several longitudinal ridges running 

 about one-half of the distance down anterior face, of which two or three bifurcate. 

 Surface of valve with rather strong concentric striae. [? Brachial vah^e nearly flat, 

 transversely sub-circular, with small pointed inconspicuous beak and thin median 

 internal ridge ; surface ornamented with rather strong concentric strise.] 

 Dimensions. — 



Brachial valve. 

 (2) 

 5 "5 mm. Length . . 4'3 mm. 



3'4 mm. Width . . 4'6 mm. 



Horizons. — (l) Stinchar Limestone Group ; (2) Balclatchie Group. 



Localities. — (l) Craighead ; (2) Ardmillan, Balclatchie, Dow HilL 



Remarks. — The type of this shell is from Craighead, where the best-preserved 

 specimens occur. Davidson's unfortunate figures of Acrotreta Nicholsoni have been 

 the cause of much confusion. Acrotreta Nicholsoni has a much lower pedicle-valve, 

 and the posterior face and interior have different characters. 



Our Craighead shells and some from Balclatchie and Ardmillan agree much more 

 closely with Conotreta Rusti, Walcott,* of the Trenton Formation, not only in their 

 external shape and features but in the internal impressions in the pedicle-valve. 

 Davidson's figure of the Ardmillan specimen is misleading, for there are no such 

 grooves on the posterior face as he has indicated 



The brachial valve of the genus Conotreta is not described by Hall and Clarke, 

 and the small flat valves above described as probably belonging to C. conoidea may 

 possibly belong to Acrothele. 



* Walcott, Proc. U.S. Nat. Mus., xii, 1890, p. 365, figs. 1-4 ; Hall and Clarke, op. tit., Brach., i, p. 104, 

 pi. ivK, figs. 16-21. 





Pedicle-valve 





(1) 



Height 



. 5'0 



Width 



. 37 



