ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 843 



ribs and interspaces equal and all crossed by concentric scale-like lamellae. Ribs 80-100 

 in number on margin of specimen 20 mm. wide, and plainly marked inside shell, 



Dimensions. — Width (average), 12-20 mm. ; length (average), 9-15 mm. 



Horizons. — (l) Drummuck Group (Starfish Bed) ; (2) Whitehouse Group, 



Localities.— (1) Thraive Glen ; (2) Shalloch Mill. 



Remarks. — M'Coy's # diagnosis of the specific characters was too brief and 

 inadequate. Davidson's figures and description are not satisfactory, and he gave 

 no details of the internal characters. 



This species occurs chiefly at Thraive, where it is abundant. It is closely allied 

 to 0. balclatchiensis, Dav., of the Balclatchie Beds, from which it principally differs 

 by its finer and more numerous ribs, by the absence of the vascular and genital 

 markings in the interior, and by the more rounded muscular scar in the pedicle- 

 valve. It is closely allied to O. insculpta, Hall, t The external and internal 

 characters show that it is referable to the subgenus Hebertella. The Ardmillan 

 specimen figured by Davidson (Suppl., pi. xiii, fig. 8) as O. erispa is referable to 

 O. balclatchiensis, and so are probably the Tramore shells recorded by Davidson. 



Orthis (Hebertella) Lapworthi, Davidson. 

 (Plate VIII, figs. 8-17.) 

 ] 883. Orthis Lapworthi, Davidson, Mon. Brit. Foss. Brach., vol. v, Silur. Suppl., p. 176, pi. xiii, figs. 9, 10. 



The specimen of this species first figured by Davidson {pp. cit., fig. 9) is said to 

 be from Shalloch Mill in the explanation of the plate, but the actual shells which 

 correspond with this figure and fig. 10, and which were marked by Davidson him- 

 self as figured, come from the Balclatchie conglomerate. None of the Shalloch Mill 

 shells attributed by Davidson to this species and thus labelled in Mrs Gray's collec- 

 tion were marked by him as figured, nor do any of them agree with his figures. 

 There is therefore probably a mistake as to the locality in the explanation of the 

 plate, and the Balclatchie conglomerate specimens must be regarded as the types 

 and as fixing the specific characters. Unfortunately, both Davidson's figures and 

 description are unsatisfactory. Thus he says that the pedicle-valve is very convex ; but 

 it is simply deeply conical, with the beak forming the apex and the hinge-area at right 

 angles to the valve ; the middle rib is also only slightly larger than the others in the 

 type-specimen and in the other better-preserved examples, but a slight median carina- 

 tion of the valve sometimes causes the middle rib to be rather more prominent than 

 the rest. The ribs are really of equal or subequal size. Davidson's fig. 10 accurately 

 depicts the brachial valve, and one internal cast of this valve shows a small cardinal 

 process, a thickened hinge-plate, small dental sockets, and stout strong crural plates. 



This species much resembles Orthis monticula, Salter,} from the Ordovician of 



* M'Coy, Syn. Silur. Foss. Ireland, p. 29, pi. iii, fig. 10. t Hall, Palxont. N. Y., i (1847), p. 125, pi. xxxii, fig. 1 2. 

 t Salter, Palxont. Niti (Calcutta, 1865), p. 43, pi. iv, fig. 7; Reed, "Ordov. Silur. Foss. Centr. Himalayas" 

 {Palxont. Indica, ser. xv, vol. vii, No. 2, 1912), p. 27, pi. v, figs. 1-5. 



TRANS. ROY. SOC. EDIN., VOL. LI, PART IV (NO. 26). 122 



