„ ^ OSTRACODA. 687 



Bythocypris cylindrloa.] 



Respecting the species about to be described and those which I have in previous 

 papers referred to Bythocypris, it is sufficient to say that in nearly every instance 

 they agree closely with one or another of the species which Prof. Jones has placed 

 under the genus. 



It may be well to call attention to the fact that the Silurian genus Cytherellina, 

 Jones and Holl,* is founded upon species very similar externally to some of the 

 Silurian Bythocyprides. Whether any of the latter have the obscure internal 

 thickenings of the test which are said to characterize Cytherellina is unknown, but 

 considering the similarity of their external features, it seems a little strange that 

 Prof. Jones has not remarked upon it in his more recent writings. 



Bythocypris oylindeica Hall. 



PLATE XLIV, FIGS. 29-35. 



Leperditia (Isochilina) cylindrica Hall, 1872, Twenty-fourth Eep. St. Cab. N. Y., p. 231, pi. viii, 

 flg. 12; Hall and Whitfield, 1875, Pal. Ohio, vol. il, p, 101, pi. iv, flg. 5. 

 (Figured in reversed position.) 



Isochilina cylindrica Miller, 1875. Cin. Quart. Jour. Sci., vol ii, p. 351. 



Bythocypris cylindrica TJlrich, 1889. Contri. Can. Micro.-Pal., p. 2, p. 48. (Not pi; ix, flg. 6.) 



Primitia minuta (part.) (Eichwald) Jones, 1890. Quart. Jour. GeoL Soc, vol. xvi, p. 7, pi. in, flgs. 

 18 and 19; not flgs. 21—23. 



Size.— Length 1.30 mm.; hight 0.65 mm.; thiclcness 0. 5 mm. 

 Length 0.71 mm.; hight 0.32 mm.; thickness 0.23 mm. 



As the characters of this species have been quite generally misinterpreted, I 

 have taken the trouble to illustrate them as far as shown in three typical examples. 

 Of the two series of measurements given above, the first may be regarded as a fair 

 average for fully grown specimens, while the other is taken from one of the smallest 

 seen. The length usually varies between 1.0 and 1.2 mm., and occasionally it 

 reaches 1.5 mm. Figures 29, 32 and 34, though differing as much in their outlines 

 as any in hundreds of valves, are but little unlike each other, and thus prove, in this 

 respect at any rate, the constancy of the species. The greatest variability noticed 

 is a slight one in the relative degree in which the central third of the ventral slope 

 is flattened or hollowed out. It is never much, yet always distinguishable. The 

 valves are slightly unequal, the left, being the larger, overlapping the right on both 

 the dorsal and ventral margins. 



On the inner side of the valves (fig. 32) a subcentral thickening of the test is 

 noticeable. Though slight, it covers considerable space, especially in its vertical 

 extent, and is of such a nature that it would cause a shallow vertical furrow on casts 



* Ann. Mag. Nat. Hist., ser. 4, vol. lii, 1889. In this paper tlie autliors redesoribe tlie (^pe species, siliqu.a, which Jones 

 had in 1855 described from casts of the interior as a Beyrieliia, and the new varieties grandis, Ursa and ovata. The last is 

 similar to Bythocypris mrtaof this report, but is not so equilateral, having the anterior end more produced ; while the typical 

 form of 0. siMgita greatly resembles B. cylindrloa Hall, sp. 



