312 Frederick Chapman: 



Dimeniiions. — Width, about 13 luin. ; lenpth of dorsal, about 

 8 iniu. ; lenjjth of ventral valve, about 9 uiin. 



Xote. — One example of a pedicel valve accompanies these 

 specimens, which otherwise are very uniform in size, and which 

 perhaps may be regarded as a senile form of the species. It 

 measures about 17 x 20 mm. ; the costae are slightly finer and 

 more numerous, about 3U, Init are of the same character as 

 those on the smaller and more typical examples. This shell 

 has a broad, shallow sinus. 



Ohservations. — These specimens from near Roan Horse Gully 

 were pi'eviously confused with Plati/stmjjJiia hiforata, Schlo- 

 theim sp., on account of their strongly spiriferoid shape and 

 stout costae with bifid character. i Their occurrence, moreover, 

 in a limestone of imcertain age, resembling the Deep Creek 

 limestone, which had already yielded that species, seemed to 

 support that conclusion. Owing to the discovery of an un- 

 doubted Upper Cambrian fauna in limestones associated with 

 the brachiopod bearing rock, these fossils have been re-examined 

 and further cleaned of matrix. The group Plectorthis, to which 

 these brachiopods belong, is characterised by having Ijiconvex 

 shells and strongly plicate valves. This group is hai'dly to be 

 distinguished from certain members of the feebler costate forms 

 of Platystrophia, excepting in the longer cardinal process, Avhich 

 in the latter is short and stout. 



Affinities. — The present spec'es bears relationship to several 

 British forms, notably of Lower Ordovician and Middle and 

 Upper Cambrian facies. In the transversely elongate form of 

 the valves and their extnided cardinal angles, comparison may 

 be made with Orf/iis (iJata, Sowerby.2 The character of the 

 costate ornament is alji'ed to that of 0. hirlsii (Salter). David- 

 son,3 i3ut, the ribs are not so numerous as in our species ; whilst 

 the latter has a smoother shell-surface. The fold and sinus is 

 not so pronounced as in Fhttystr()p]}ia hiforata. Schlotheim sp.,"* 

 although the large example previously mentioned approaches it 



1 F. Chapman. " On Some Fossils from .Siluiian r.imostoMes, DolodrooU Valley, Mt. 

 Wellin^on, Victoria." Vict. Nat., vol. xxiv., .Juno, 190", !>. 34. 



2 Davidson. Mon. Brit. Sil. Brach., No. 3, lS(ii), p. i'M jil. xsxiii., tiys. 17-21. 



3 Tom. supra oit., p. 230, pi. xxxiii., fiy:s. 13-l(i. 



4 See Davidson. .Mon. Brit. Sil. Braeh., No. 4, 1S71, )>. 2(i8. pi. xxxviii.. ti;;s. 11-2.'); 

 Hall and Clarke, I'al. of .New ^ork I'al. Bracli., jit. i. ls!)2, p. 2(10, pi. \.h, fiys. I-lo. 



