OSTRACODA. 687 
Bythoeypris cylindrica.] 
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 cytinprica Hall. 
PLATE XLIV, FIGS. 29—35. 
Leperditia (Isochilina) cylindrica HALL, 1872, Twenty-fourth Rep. St. Cab. N. Y., p. 231, pl. vi, 
fig. 12; HALL and WHITFIELD, 1875, Pal. Ohio, vol. ii, p, 101, pl. rv, fig. 5. 
(Figured ip reversed position.) ; 
Tsochilina cylindrica MILLER, 1875. Cin. Quart. Jour. Sci., vol ii, p. 351. 
Bythocypris cylindrica UuLRicH, 1889. Contri. Can. Micro.-Pal., p.2, p. 48. (Not pl. 1x, fig. 6.) 
Primitia minuta (part.) (EICHWALD) JONES, 1890. Quart. Jour. Geol. Soc., vol. xvi, p. 7, pl. 11, figs. 
‘ 18 and 19; not figs. 21—23. 
SizE.—Length 1.30 mm.; hight 0.65 mm.; thickness 0.5 mm. 
Length 0.71 mm.; hight 0.382 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 sucha nature that it would cause a shallow vertical furrow on casts 
“ * Ann. Mag. Nat. Hist., ser. 4, vol. iii, 1869. In this paper the authors redescribe the type species, siliqua, which Jones 
had in 1855 described from casts of the interior asa Beyrichia, and the new varieties grandis, tersa and ovata. The last is 
similar to Bythocypris curta of this report, but is not so equilateral, having the anterior end more produced; while the typical 
form of O, siliqua greatly resembles B. cylindrica Hall, sp. 
