Monthly, Microscopical) Qstracodous and Phyllopodous Tribes. 191 
carapace, it was probably the case also with the internal organs ; 
and after all, whatever classification we may adopt for such extinct 
types, it can only be provisional. 
2. Isochilina (Fig. 18).—Some Lower Silurian Entomostraca 
from Canada, less than half the size of Leperditia Balthica, and dif- 
fering from Leperditia chiefly in being equivalved, the margins of 
the valves meeting uniformly and not overlapping, were separated 
by me in 1858 as belonging probably to a sub-genus.* They are 
possibly members of a distinct genus. There were two species 
noticed, Isochilina Ottawa and I. gracilis (see pl. 10, figs. 1 and 2, 
loc. cit.). 
3. PPrimétia Solvensis (Jones) and P. (?) punctatissema (Salter, 
MS.) are the oldest Bivalved Entomostraca as yet recorded. They 
are from the Lingula-flags, or lowest division of the great Silurian 
system of strata. Both of these little fossils were at first regarded 
as minute Leperditizx, on account of a resemblance in shape; and 
several of their allies, from other old Silurian rocks of Wales and 
Canada, were in 1855 grouped as representing the simplest type of 
Beyrichia, the characteristic furrowing being absent except the 
apparently essential nuchal sulcus. In 1865, however, these simple 
or unisuleate forms (Beyrichix simplices of 1855) were separated 
generically, because among the increased number of specimens we 
found a persistent occurrence of the chief features, with a passage 
towards Leperditia, by the loss of the furrow, rather than towards 
the bisulcate or real Beyrichix.t The Primitiz have a minute 
bivalved carapace, equivalved or nearly so, convex, more or less 
oblong, often approaching Leperditia in shape by the sloping of 
the dorsal angles; hinge-line straight, sometimes nearly as long as 
the valve. The surface of each valve is usually impressed on the 
dorsal region, either medially or towards the anterior extremity, 
with a vertical sulcus, variable in size, sometimes barely visible, 
sometimes passing into, or even merely represented by, a nayel-like 
pit; and sometimes the sides of the sulcus are swollen, and even 
raised up into tubercles. The ventral edges of the valves in some 
old individuals bear well-defined and even double marginal keels. 
A list of all the known Primitiz is given in ‘Ann. Nat. Hist.,’ 
ser. 4, vol. xvi., p. 222, &e. There have been thirty-nine species 
as yet recognized; thirty-three of these were met with in the 
British Isles. The genus appears to be limited to the Silurian 
period (see Fig. 14). 
4, The Beyrichiz (Figs. 19-21) are very abundant throughout 
the Paleozoic series, from the lowest Silurian to the Carboniferous 
strata. They have small oblong or semi-ovate valves, strongly lobed 
* ‘Ann. Nat. Hist.,’ ser. 3, vol. i., p. 248. 
+ Ibid., ser. 2, vol. xvi. pp. 85, 90, 171, &e. 
t ‘Ann. Nat. Hist.,’ ser. 3, vol. xvi., p. 144. 
