257 



larger, wUli (lie depression in tlie mecliiin fold and the plication in 

 tlie median sinus well defined. Thus we have forms which approach 

 so closely those from Erere, that it is impossible to separate the two. 

 They are undoubtedly identical. 



Obtained in great abundance from the Devonian sandstone of 

 Erere, associated with Spirifera Pedroana, etc. A few specimens 

 have also been found in the yellow shale underlying the sandstone. 

 (Morgan Expeditions 1870 and 71.) 



Discina lodcnsis, Hall. 



Orhicula lodensis, Hall, Geol. Rep. Fourth Dist. N. Y., p. 223. 

 Or'bicula lodensis, Vanuxem, Geol. Rep. Third Dist. N. Y., p. 168. 

 Discina lodensis, Hall, Pal. N. Y., Vol. IV, p. 23. 

 Compare Discina media. Hall, Pal. N. Y., Vol. IV, p. 20. 



Test of the Erere variety small, subcircular or broadly subovate in outline ; 

 breadth about equal to the length, and greatest at, or slightly anterior to, the 

 middle ; generally narrowing more or less posteriorly, and often slightly trun- 

 cate behind. 



Both valves are always so flattened in the shale, that their true convexity is 

 undeterminable. The nuclei, which were probably acute, at least in the dorsal 

 valve, are also flattened down to the surface of the valves and broadened. 

 Their distance from the posterior margin varies with the age of the test, it 

 being, in the dorsal valve, from about one-third the length of the valve in the 

 very young, to about one-fifth the length in full grown specimens ; but in the 

 ventral valve, their distance from the posterior margin is always somewhat 

 greater than in the dorsal valve, averaging- about one-third the length of the 

 valve, thus giving more space for the foramen, which is narrow, linear, and 

 extending from very near the nucleus to within a varying distance from the 

 posterior margin. The concentric lines of growth are numerous, rather fine, 

 yet prominent, and more closely crowded together posterior to the nuclei. 



The larger specimens measure in length and breadth about 8 m. m., and 

 there are all sizes from this down to individuals of very small size. 



This Discina from Erere, is undoubtedly only a variety of Discina 

 lodensis, Hall, Genesee shale, New York. The Brazilian variety, 

 together with D. lodensis and D. media, Ilall, of the Hamilton and 

 Chemung groups, probably forms a single species, which extends 

 through all the latter portion of the Devonian age of North Amer- 

 ica, and has also a wide geographical range ; the specimens varying 

 somewhat, according to the conditions under Avhich they lived, and 

 the rock in which they are preserved. 



BUL. Brr. soc. nat. sci. (33) januart, 1874. 



