SILURIAN OSTRACODA FROM NORTH AMERICA, ETC. 541 



like, ridges ; and therefore it comes nearer to S~ sigmoidalis, J. 

 (' Quart. Journ. Geol. Soc' February 1890, p. 11, pi. ii. fig. 4) than 

 to those figured in the 'Ann. & Mag. iS'at. Hist.' May 1886, 

 pp. 403-406, pi. xiii. figs. 1-9 & 15. Indeed it came from the 

 same locality as S. sigmoidalis, namely from the Hamilton Group at 

 Eighteen-mile Creek, Lake Erie, N.Y. Coll. Dr. G. J. Hinde, 

 F.G.S. 



The valve has unequal ends, being semicircular in front and ob- 

 liquely rounded behind. In outline it is somewhat like the sole of 

 the human foot. Hence it may be called plantaris. The broad or 

 hinder end has a thin, flat, lip-like margin, obscurely marked with 

 about ten shallow pits. The narrow or front end has six strong, 

 outstanding, marginal denticles. There is a thick curved ridge 

 within the posterior margin. The surface bears a thin, sigmoidal, 

 tortuous ridge, passing along the dorsal and along the middle and 

 ventral regions, curving in front and behind. It is double for a 

 short distance in the antero-dorsal region, and is slightly interrupted 

 at the anterior third of the valve. It gives off two short branches 

 in the posterior third of the valve ; the higher one touches the in- 

 side curve of the thick ridge, and the lower branch falls into the 

 ventral end of that ridge. 



YI. OcTONARiA, Jones, 1887. 

 Octonaria, Jones, 'Ann. & Mag. Nat. Hist.' June 1887, p. 404. 



10. Octonaria Linnarssoni, sp. nov. (PI. XX. figs. 7 a, 7 h.) 



Length 1*4 mm. ; height '7 mm. 



In this specimen we have the two very convex and thick-ridged 

 carapace-valves of Octonaria, but with a surface -pattern different 

 from any of those described and figured in the ' Annals & Mag. ]^. 

 Hist.' ser. 5, vol. xix. pp. 404-407, pi. xii. figs. 1-8. In shape it 

 approaches O.fleocuosa, loc. cit. fig. 1 (if its position on the plate be 

 reversed), but not at all in the ornament. This is a compressed 

 spiral ridge, beginning with a thick portion at one end, becoming 

 thinner as it passes along the ventral region, and much thinner 

 after curving up into the dorsal region, then throwing off a short 

 connection with the first portion, and turning down into the middle 

 of the valve with a confused loop, little lumps, and central pit. 

 The last feature can be traced obscurely in figs. 2, 5, & 7 of the 

 illustrations referred to. Although, perhaps, some doubt attaches 

 to the determination, I think that both naturally and conveniently 

 it may be referred to Octonaria as 0. Linnarssoni — after the 

 lamented Swedish palseontologist who has done good work among 

 these and other fossil Crustacea. 



From the Hamilton Group, Clarke Co., Indiana. Coll. Mr. J. M. 

 Clarke, of Albany, IS'.Y. 



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