Mr. T. R. Jones on Beyrichise. 169 



4. Beyricnia Bussacensis, Jones. rJ. VI. fig. ^^ic^^mjo,^ 

 Quart. Journ. Geol. Soc. vol. ix. p. 160. pi. 7. figs. 5/6.' '' 



Surface of the valve depressed, smooth, bearing three well- 

 defined, transverse, slightly curved, narrow, separate ridges; 

 posterior ridge close to the posterior margin, and curving down- 

 wards and forwards until it meets and runs into the marginal 

 rim of the ventral and anterior borders. Marginal rim well 

 developed, and raised into a narrow continuous ridge, which in 

 old specimens is one and the same with the posterior ridge 

 and its extension forward. 



In the majority of adult specimens (for instance, fig. 5. pi. 7, 

 Quart. Journ. Geol. Soc. vol. ix.), the posterior ridge is placed 

 close to the posterior margin ; but in the young state (fig. 6. loc, 

 cit.) and in the specimen here figured (fig. 14) a shallow de- 

 pression occurs behind this ridge. 



In young individuals the marginal rim is not so strongly 

 developed, and the valve is rather less quadrate in outline. 



This characteristic species occurs in great numbers on the 

 divisional planes of Lower Silurian schists from near Coinibra 

 (Serra de Mucela, and Porto de Louza in the Serra de Bussaco), 

 Portugal, which form part of the collection made by Senhor C. 

 Ribeiro and described by Mr. D. Sharpe, Quart. Journ. Geol. 

 Soc. vol. ix. pp. 135 et seq. oo^ 



. (Corrugate .) dns 



Wil^:^- Beyrichia Ribeiriana,now. %^. PI. VI. fig. 15. ■{ 



^Carapace-valves contracted anteriorly, convex, impressed to- 

 wards the dorsal border by two short, broad furrows, the hinder 

 of which is largest and subcentral. The convexity of the valve 

 forms a broad curved posterior lobe, a narrow short, oblique 

 middle lobe, and an oblique anterior lobe ; the last two near 

 together, and forming a short angle, with the apex pointing 

 downwards and backwards ; and all three lobes continuous with 

 the convex ventral portion of the valve. The posterior lobe is 

 frequently indented on its broad dorsal extremity. Marginal 

 rim indistinct. 



The younger specimens may be described as presenting a 

 nearly semicircular convex lobe, parallel with the ventral border 

 and bounding a subcentral pit or furrow ; the extremities of the 

 two arms of the lobe being each, but unequally, impressed by an 

 obliquely vertical indentation. 



This interesting form (which I have named after Senhor C» 

 Ann, ^ May. N. Hist. Ser. 2. Vol.xv'i. 12 



