Paleozoic Bivalved Entomostraca. 347 
Pl. XIX. fig. 12 exhibits the outlines of an impression (in- 
side of a hollow cast) of the outside of the fore part of a left 
valve, which was probably 3; inch long when perfect. It is 
in white Upper-Silurian Porambonite Limestone, from Gat- 
schina, near Saretsche, in the Government of St. Petersburg. 
The rock is crowded with similar specimens, as convex and 
concave casts. It was sent to me (with other specimens) in 
1862, by the late M. d’Hichwald, and is the same as the 
fragments of white Lower-Silurian Limestone with Entom- 
ostraca which I treated of in the Ann. & Mag. Nat. Hist. 
1856. The figs. 11 and 12 of pl. vil. in that volume, and re- 
ferred to at pp. 91 and 100, belong to this species, and not to 
L. marginata® (Keyserling). I find that one, at least, of the 
specimens there concerned shows, under the microscope, casts 
of those minute pits on the marginal flange of the ventral 
border which originated d’Hichwald’s “ L. phaseolus, var. 
punctata,” and Schmidt’s ‘ Jsochilina punctata.” 
Fig. 12 is an enlarged drawing of the hollow impression of 
the antero-dorsal angle (with its eye-tubercle) of a left valve, 
similar to fig. 12,a, pl. vu. Ann. & Mag. Nat. Hist. loc. cit., 
and belonging to J. punctata, Schmidt. 
This species has its North-American representative in the 
Lower-Silurian Lsochilina gracilis, Jones, of Canada (Ann. & 
Mag. Nat. Hist. 1858, ser. 3, vol. 1. p. 248, pl. x. fig. 2; and 
Geol. Surv. Canada, decade ili. p. 98, pl. x1. fig. 15). 
2. Isochilina grandis (Schrenck). 
The large specimen from Rupert’s Land, described and 
figured as ‘ Leperditia marginata, Keyserling?” inthe Ann. 
& Mag. Nat. Hist. ser. 2, 1856, vol. xvi. pp. 94 and 100, 
pl. vii. figs. 14, a-d, is an Isochilina. It differs materially 
from Keyserling’s “ Cypridina marginata,”’ as pointed out 
by Dr. F. Schmidt in his memoir on Russian Leperditie, 
1873, p. 19. It is related to Isochilina formosa, Barrande 
(Syst. Sil. Bohéme, vol. i. suppl. 1872, p. 534, pl. xxiii. 
fies. 22-25, and pl. xxxiv. figs. 1-3), and to Jsochilina gigan- 
tea (Romer), described and redrawn by M. Barrande, op. cit. 
p. 5385, pl. xxxiv. figs. 4-6. It differs from both sets of 
figures in the slope of its antero-ventral region, its obliquely 
ovate body, and the more elliptical curve of its lower margin. 
It still more closely resembles, however, Schmidt’s figures of 
Schrenck’s “ Cypridina grandis” (Russ. sil. Leperd. figs. 1, 
3-6), to which species Schmidt refers Ferd. Rémer’s “ Leper- 
ditia gigantea” above mentioned. This last is determined 
by M. Barrande to be an Isochilina; and although Dr. 
* As pointed out by F, Schmidt, Russ. sil. Leperd. 1873, p. 22. 
