BRITISH SILURIAN BRACHIOPODA. 185 



almost impossible to obtain good specimens. The rock is of a dark-grey colour and 

 exceedingly tough and hard, and the specimens it contains are in a very fragmentary 

 condition, chiefly casts or impressions. We have no evidence of the presence of 0. 

 Iliclcsii in Anglesea. 



104. Orthis retrorsistria, 31'Goi/. Dav., Sil. Mon., PI. XXXVI, figs. 39—42 ; and 



PL XXXI, figs. 2, 4, 5, and 6, belong to 0. 

 retrorsistria ; 1, 3, and 7 to 0. alternata, Sil. 

 Sup., PL XIV, figs. 7—16. 



Orthis retrorsistria, M'Coy. Pal. Foss., p. 224, pis. i,'figs. 12, 13, 1852. 



— ALTERNATA, Salter (not of Sowerbj'). Memoirs of the Geol. Survey of Great 



Britain, p. 340, 1866. 



— — Salter (not Sow.). A Catalogue of the Collection of Cambrian 



and Silurian Fossils in the Museum of 

 the University of Cambridge, p. 60, 1873. 



When writing my ' Silurian Monograph,' I was acquainted with M'Coy's 0. 

 retrorsistria from that author's description and figures only, and so had to limit myself 

 to the reproduction of his description and illustrations. Blindly coinciding in the mistaken 

 view entertained by Mr. Salter, I described Orthis retrorsistria as a variety of 0. alter- 

 nata, and in PL XXXI figured under the last name specimens of the two species. 



In January, 1880, T. M'Kenny Hughes, Professor of Geology in the University of 

 Cambridge, having in the most liberal manner forwarded for my inspection the extensive 

 series of specimens of 0. alternata and 0. retrorsistria in the Cambridge Woodwardian 

 Museum, I arrived, after a searching and careful examination, at the conclusion that 

 Sowerby's and M'Coy's species were specifically distinct. 



Salter, at p. 340 of the ' Memoirs of the Geological Survey of Great Britain,' 1866, 

 considers 0. retrorsistria a synonym of 0. alternata ; and at p. 60 of the Cambridge 

 Catalogue, above quoted, when alluding to 0. alternata, Sow. (' Siluria,' 2nd ed., 

 pi. 6, fig. 5), says : " There can be no doubt the Welsh fossil 0. retrorsistria is the 

 dwarf form of the common Horderley species." 



I have, however, now had the advantage of being able to study several hundred 

 specimens of all ages and shapes of 0. retrorsistria, which abounds in many localities, so 

 abundant, indeed, that on a slab in the Cambridge Museum, measuring about eighteen 

 inches by twelve, from the Middle-Caradoc Shale of Cerrig-y-Druidion, more than a 

 hundred specimens can be counted on its surface, and in an excellent state of 

 preservation, although in some instances slightly out of shape ; and Mr. Salter says it 

 occurs there in millions. Specimens identically similar are also in the Cambridge 



