. Fresh-water biology. Freshwater biology. 640 FRESH-WATER BIOLOGY 20 (19) Ventral margin of the branchial area without long processes. Dero Oken 1815, D. limosa Leidy is abundant and the best known of the North American species. D. oblusa d'Udekem and a species which prob- ably is D. perrieri Bousefield are of frequent occurrence, but a careful study of the North American representatives of this genus, as well as of Nais, is necessary before we can be sure of their exact relation to European species. Fro. 986. Posterior end of Dero limosa. X 25. (After Bousefield.) First anterior dorsal setae
. Fresh-water biology. Freshwater biology. 640 FRESH-WATER BIOLOGY 20 (19) Ventral margin of the branchial area without long processes. Dero Oken 1815, D. limosa Leidy is abundant and the best known of the North American species. D. oblusa d'Udekem and a species which prob- ably is D. perrieri Bousefield are of frequent occurrence, but a careful study of the North American representatives of this genus, as well as of Nais, is necessary before we can be sure of their exact relation to European species. Fro. 986. Posterior end of Dero limosa. X 25. (After Bousefield.) First anterior dorsal setae on II 29 Dorsal setae of two kinds: capilliform and shorter needle-form setae which commonly have cleft distal ends. Naidium O. Schmidt 1847. N. osborni Walton has been described from Lake Erie (Walton, 1906). This genus is united with Prislina by some writers. 23 (22) Dorsal setae all capilliform, mostly with very fine teeth on convex side; prostomium commonly elongated into a proboscis. Pristina Ehrenberg 1831. P. longisela var. leidyi Frank Smith has the capilliform setae of III greatly lengthened (.700 mm.) and without serrations. The typical form of this species as found in Europe has extremely minute inconspicuous serrations on the capilliform setae of dorsal bundles of somites other than III. In the variety P. I. leidyi, which is found in the United States and certain other parts of the world, the serrations are coarser and more easily seen. Fig. 987. Pristina longiseta leidyi. c. s., ventral seta; X300. i. j., part of seta from dorsal bundle. X 4So. (After Smith.) P. flagellum Leidy has a very characteristic posterior end. Specimens of this species have been met with by the writer but once and when there was no opportunity for study beyond enough to convince him of the general accu- racy of Leidy's description and that the species really belongs to Prislina. Representatives are sometimes foimd of certain species in which the dorsal setae of III are not especially
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