828 



SYSTEMATIC HISTOEY OF THE INFUSORIA. 



this genus from Cj'clotcUa is the areolation of the secondary surfaces" 

 (Mencg.). ^' This genus finds its nearest allies in the Melosireae, whose 

 genera, from their filamentous character, stand widely apart. Were the 

 frustules of Coscinodiscus eccentricus, for example, permanently coherent after 

 self- division, it would be difficult to separate them, in a generic point of 

 "\iew, from those of OrtJiosira nivalis, which have the same cellular structure, 

 or from those of Melosira aurichcdcea or M. sulcata, which are fiumished 

 with a projecting fringe of silex, the homologue of the spinous processes in 

 C. eccentricus^' (Smith, BD. i. p. 23). Coscinodiscus is easily distinguished 

 from most genera in this family by a disk not di^dded into compartments. 

 In the greater number of species the cellules have a radiating arrangement, 

 and become smaller near the margin ; the former character, however, is fre- 

 quently obscure, and is best seen by as low a magnifying power as will suffice 

 to determine the cellular or dotted structure. 



* Disc ivith a few centred larger {gene- 

 redly oblong) celhdes, steUeitely eirranged, 

 and forming an umhiliccd rosette (^rim 

 striated). 



t Disc large, and its cellules distinct. 



Coscinodiscus centredis (E.). — Cel- 

 lides minute, nearly equal, in crowded 

 radiating series; umbilical rosette of a 

 few oblong celliiles round a circular one. 

 EM. pi. 18. f. 39 ; GDC. p. 28, pi. 3. f. 50. 

 Fossil. Virginia and Sicily. A large 

 species with striated rim. 



C. omplicdanthus (E.). — Cellides in 

 radiating series, marginal ones smaller, 



7 to 8 in 1-1200", middle ones larger, 

 6 in 1-1200" ; umbilical rosette of 7 or 



8 large oblong cellules. KSA. p. 125. 

 Bermuda deposit. Disc large. D. 1-96". 

 Mr. Briglitwell finds it difficult to distin- 

 guish this species from the foUowdng, 

 and considers their specific characters 

 unsatisfactory. 



C. Oeidus Iridis (E.). --Cellules hex- 

 agonal, in radiating series, smaller at 

 the margin and near the umbilical rosette, 

 which is formed of from 5 to 9 large 

 oblong cellules. EM. pi. 18. f. 42. Fossil 

 and recent. America, Europe, Milford 

 Haven, &c. This large species, when 

 dry, is marked with colom-ed rings, — an 

 effect apparently due to the peculiar 

 arrangement of its cellules. It differs 

 from C. centralis in its larger cellules, 

 and from C. asteromphcdus by the absence 

 of a veil. ^' This species, both in the 

 recent and fossil specimens, often ac- 

 quires a size not much inferior to that 

 of C. gigas'^ (Bailey). 



" C. horealis (Bail.). — Disc having at 

 its depressed centre a conspicuous star, 

 formed of about 6 large cellides. _ The 

 rest of the sm'face covered with inter- 

 ruptedly radiant lines of prominent hex- 



agonal ceUules, which increase regularly 

 from near the centre to the convex 

 margin." B. in Amer. Jom-n. of Science 

 and Ai'ts, 1856. Sea of Kamschatka. 

 '■' This resembles C. Ocidus Iridis ; but 

 the cellules forming the star are more 

 rounded, and the other cellules are 

 larger " (Bailey). 



C. asteromjjhalus (E.). — Cellides in 

 radiating series, smaller towards the 

 margin; umbilical rosette distinct ; sm- 

 face appearing as if covered bv a very 

 finely punctated veil. EM. pi. 18. f. 45. 

 Fossil. America. Cellides large, rather 

 tumid. C. asteromphalus difters from the 

 other species with stellate mnbilicus by 

 its minutely punctated cellides. 



2 1 Disc with cellules obscure, and re- 

 quiring the higher magnifying powers 

 to discern them. 



C. concinnus (Sm.). — Disc large, with 

 radiating series of minute puncta, and 

 an umbilical irregidar rosette of larger 

 cellules, di\dded into compartments by 

 radiating lines, which terminate at the 

 margin in minute spines, (v. 89.) SBD. 

 ii. p. 85; Roper, MJ. vi. p. 20, pi. 3. f.12. 

 Europe. Valves convex. In some spe- 

 cimens the markings are very inconspi- 

 cuous, and difficidt to detect ; in others, 

 as in the specimens from Hidl, moi'e 

 evident. 



C. stellaris (Roper). — Disc extremely 

 hyaline, with very fine, inconspicuous 

 radiating series of puncta, and a few 

 larger, stellately arranged umbilical cel- 

 lides. (v. 83.) RoMJ. vi. p. 21, pi. 3. 

 f. 3. Caldy, Pembrokeshire. When 

 mounted in balsam, the disc is so hya- 

 line, and the pimcta so difficult to detect, 

 that it is liable to be regarded as a 

 detached ring. Dry valves brownish, 

 without marginal spines. 



