858 DR F. R. C. REED ON THE 



Locality. — Thraive Glen. 



Remarks. — There is only one internal cast of the pedicle-valve and a portion of 

 the external impression of the same specimen which I have observed amongst 

 the collections. But it is such a well-marked and peculiar form that it certainly 

 deserves attention. It must be referred to the subgenus RJiipidomella, and is un- 

 doubtedly allied to 0. polygramma, though the muscle-scars are different in that 

 species. It more resembles some Devonian or Carboniferous species of Rhipidomella, 

 and though at present imperfectly known it may be distinguished by the specific 

 name 'prematura. The American species Rhipidomella magnicardinalis, Foerste,* 

 from the West Union Limestone (Silurian) of Ohio, appears to be allied to this 

 Girvan form. 



Subgenus schizophqrella, nov. 



Definition. — Shell unequally biconvex, transversely elliptical, with hinge-line less 

 than width of shell and cardinal angles obtuse, with a more or less independent 

 broad low fold on the brachial valve and corresponding sinus on the pedicle-valve. 

 Beaks and hinge-areas on both valves small. Surface of shell covered with numerous 

 small closely placed thread-like radiating lines. Interior of pedicle-valve with large 

 elongated well-marked muscle-scars parallel or divergent, and thin short dental plates. 

 Interior of brachial valve with prominent oval or lanceolate cardinal process and 

 stout prominent curved crura. Ovarian markings in both valves frequently strong. 

 Type : Orthis fallax, Salter. 



The species comprised in this new subgenus appear allied to the Schizophoria 

 group f of Upper Palseozoic times, and cannot well be referred to any of the preceding 

 subgenera. 



Orthis (Schizophorella) fallax, Salter. 



(Plate X, figs. 19-24.) 



1846. Orthis fallax, Salter, Appendix to M'Coy's Syn. Silur. Foss. Ireland, p. 72, pi. v, fig. 3. 



1868. Orthis fallax, Salter, Davidson, Mon. Brit. Foss. Brach., vol. iii, pt. vii, p. 223, pi. xxxi, 



figs. 9-11. 

 1883. Orthis mullochiensis, Davidson (pars), ibid., vol. v, Silur. Suppl., p. 180 (specimens from Upper 



Bala of Thraive). 



This species is very abundant in the Starfish Bed of Thraive Glen, and it agrees 

 absolutely with the typical examples of 0. fallax which Salter figured from Tyrone, 

 but it was erroneously placed with the Llandovery species 0. mullochensis \ by 

 Davidson. Though undoubtedly allied to it, 0. fallax, as M'Coy§ pointed out, differs in 

 the following points : — (l) The cardinal process is more linear ; (2) the median septum 

 is weaker and often inconspicuous ; (3) the diductors form a regular oval scar in tlie 



" Foerste, Journ. Cincinnati Soc. Nat. Hist., vol. xxi, No. 1 (1909), p. 27, pi. i, figs. 11a-d, pi. ii, figs. 12a-b. 



t Hall and Clarke, Palxont. New York, vol. viii, Brach., i, p. 211. 



% Davidson op. cit., iii. p. 221, pi. xxix, figs. 14-18, § M'Coy, Syn. Brit. Pal. Foss., p. 261. 



