ROTALIINAE 411 



338. Discorbis isabelleana (d'Orbigny) (Plate XIV, figs. 5-8). 



Rosalina isabelleana, d'Orbigny, 1839, FAM, p. 43, pi. vi, figs. 10-12. 

 Thirteen stations: 388; WS 76, 83, 84, 86, 87, 88, 91, 97, 225, 245, 246, 248. 



This is one of the largest and most handsome of the Falkland species. It is usually 

 represented by a few worn and dead shells only, but was common at WS 246 where it 

 reached a large size, and still more abundant at 388 where the specimens were variable 

 and handsome, owing to the development of tubercular ornament on the dorsal surface. 

 Equally varied, but less abundant, at WS 88 and 225. Sessile specimens were seen at 

 WS 225 and 246. There is great variation in the degree of convexity of the dorsal side, 

 which is sometimes almost flat. The dorsal surface also varies, from glassy smoothness to 

 rough, according to the amount of Hmbation and development of soHd beads of shell. 

 The ventral surface is invariably smooth and glassy, the umbilical cavity often more or 

 less filled with solid beads of shell-substance or projections from the umbilical margins 

 of the chambers. But there is always a noticeable cavity, never a solid stud of shell- 

 substance, such as is suggested by Brady's figure (B. 1884, FC, p. 646, pi. Ixxxviii, fig. i). 

 The punctation is extremely fine, not coarse, as suggested by d'Orbigny's figures 

 which are extremely unsatisfactory, and must be regarded as responsible for most of the 

 confusion which has arisen over his species. His description, on the other hand, is quite 

 good, and, taken in conjunction with the measurements which he gives as 2 mm. 

 diameter, should have obviated some of the errors which have arisen over the identity 

 of his species. 



Even so it is difficult to understand on what grounds Jones and Parker (J. & P. 1872, 

 FFR, p. 115) assigned d'Orbigny's species to the company of his Rosalina rugosa and 

 Rosalina vilardeboana, in the group of Discorbina turbo. This appears to have been the 

 initial stage of an error, which since that date has continually recurred. 



Brady, loc. cit., follows suit with a description of D. isabelleana as a "minute (italics 

 ours) thin shelled variety belonging to the rosacea group". His figure is not unlike 

 d'Orbigny's, but the umbilical cavity is filled with a solid stud of shell matter. It 

 represents a dehcate little species, 075 mm. in diameter, not uncommon in coral sands, 

 which we have recorded from several localities, following the Brady figures. 



Egger's figure (E. 1893, FG, p. 386, pi. xv, figs. 36-8) is very poor, but the description 

 and size 0-25 mm, are sufficient evidence that he was not dealing with d'Orbigny's 

 species. He himself assigns his specimens to Brady's form. 



Cushman's (C. 1910, etc., FNP, 1915, p. 15, pi. vi, fig. i) figure appears to be even 

 farther from the d'Orbignyan type. It represents a high conical shell with almost flat 

 base. The size is again given as o-25-o-40 mm. diameter. 



Later papers, as, for example, those of Cushman (C. 1927, FWCA, p. 160, pi. iv, 

 fig. 4), Cushman and Kellett (C. & K. 1929, WCSA, p. 9, pi. iii, fig. 12), Cushman and 

 Valentine (C. & V. 1930, FGC, p. 23, pi. vi, figs. 6-8) show a remarkable diversity of 

 figures, none bearing much resemblance to those of d'Orbigny, while the sizes recorded 

 and the descriptions indicate that the organisms dealt with are various forms allied to 



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