838 DR F. K. C. REED ON THE 



to this species differ to some extent from the typical form of O. flabellulum* The 

 ribs are more numerous and divide more regularly and more often, splitting up into 

 two or three smaller ones at one-third to two-thirds their length, and as there are 

 24-30 primaries the total number of ribs on the margin is much greater than in the 

 type. Indeed, the shells approach O. porcata in their ribbing and shape, but the 

 ribs are coarser and stronger, the pedicle-valve is shallower, and the beak less 

 prominent. Davidson's figures of this Girvan variety do not correctly represent 

 the ribbing. There is no good interior preserved, but a fragmentary cast of a 

 pedicle-valve shows the characteristic muscle-scar. The specimens from Craighead 

 are all of large size and measure 30-35 mm. in length. Smaller examples of the 

 same variety occur at Ardmillan Brae. We may note a considerable resemblance to 

 Dinorthis Ulrichi, Foerste,f from the Ordovician of Kentucky. 



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



Localities. — (l) Craighead ; (2) Ardmillan Brae. 



Orthis (Dinorthis) porcata, M'Coy. 

 (Plate VII, figs. 7-12.) 



1846. Orthis porcata, M'Coy, Syn. Silur. Foss. Ireland, p. 32, pi. iii, fig. 14. 



1848. Orthis inflata, Salter, var. retrorsa, Salter, Mem. Geol. Surv., vol. ii, p. 373, pi. xxvii, figs. 3, 4. 

 1852. Orthis porcata, M'Coy, Syn. Brit. Pal. Foss. Woodw. Mus., p. 223, pi. ia, figs. 41, 42. 

 1 1860. Orthis Carleyi, Hall, 13th Rep. New York State Cab. Nat. Hist, p. 120, text-fig. 

 1870. Orthis porcata, M'Coy, Davidson, Mon. Brit. Foss. Brach., vol. iii, pt. vii, pi. xxxi, figs. 12-16 



(1 17-20). 

 1883. Orthis Sowerhyana, Davidson (pars), Mon. Brit. Foss. Brach., vol. v, Silur. Suppl., pp. 78, 226. 

 1883. Orthis flahellidum, Sowerby, Davidson (pars), ibid., p. 179, pi. xiii, fig. 6 (non figs. 3-5, 20, 21). 

 1 1892. Ortliis (Plesiomys) retrorsa, Salter, Hall and Clarke, Palxont. New York, vol. viii, Brach., i, 



pp. 197, 222, pi. va, figs. 14-16. 

 ? 1912. Dinorthis Carleyi, Hall, Foerste, Ohio Naturalist, vol. xii, No. 3, pp. 449, 453, pi. xxii, 

 figs. 8a-d. 



The resupinate shells from Girvan which I refer to O. porcata were identified by 

 Davidson partly as O. flabellulum, Sow., and partly as O. sowerbyana. All of them 

 come from the Upper Bala or Starfish Bed of Thraive Glen. Davidson apparently 

 relied on the internal characters of the pedicle-valve for his identification of some as 

 O. flabellulum (Supply p. 179, pi. xiii, fig. 6), for there is a note in his writing on one 

 marked specimen saying : " This is a very characteristic interior of the ventral valve 

 of O. flabellulum, making the identification certain." But the characters of the 

 exterior of the shell are quite sufficient to separate it from this species, while 

 internally there are certain differences which show that it cannot be associated with 

 the typical O. flabellulum, Sow., from Boduan. 



Some of the Girvan shells are more convex than others, and such were labelled 

 O. sowerbyana by Davidson and quoted by him from Thraive Glen (op. cit., p. 178). 



* Davidson, Mon. Brit. Foss. Brach., vol. iii, pt. vii, p. 248, pi. xxxiv, figs. 1-12. 

 t Foerste, Bull. Denison Univ., vol. xiv, art. 17 (1909), p. 320, pi. vii, figs. 7a-c. 





