240 E. Billings on the Genus Centronella, 
being represented by a narrow thread-like ridge. In others, it 
is larger, and, in many, strongly developed. In Leptena sordida 
and L. decipiens, there is no divaricator process. In Strophomena, 
all the Lower Silurian species have a wide foramen; in the Mid- 
dle and Upper Silurian rocks, species make t eir appearance 
with a much narrower aperture, and, in the Devonian, we find 
‘many with this opening reduced to a mere line, and some wi 
it obsolete altogether. In this same series, we find also a grad- 
ual increase in the extent to which the area is striated; it peas 
smooth in the Lower Silurian, partly striated in the Middle an 
Upper one se and, in the Dev onian, sometimes, as in S. demissa, 
ornamented the whole length with transverse line 
We have are a gradual transition from S. slieeatta to S. de- 
missa, in which two species the characters of the hinge and area 
are so different that they have been placed in different genera.’ 
airertn Verneuilit has large dental pistes, but O. festinala none 
all. Spirifera Mosquensis has these plates extending more 
ie half the length of the valve, but S. mucronata is desti- 
tute of them. Almost precisely the same differences exist be- 
tween the internal characters of Zerebratula vitrea and T. elongata 
as those relied upon for the separation of C. Julia from C. glans- 
fagea. ane ip described by peewee. in I. elongata ae 
separate it from that genus ‘and make it the Soniation of i 
Cryptonella will not be successful. : 
‘ species, idalis in the D RADI Sa 
Ww wide foramen and non-striated area of S. alternata. But 
a true Lower Silurian form, which appears t sprung from the stock of 8. 
and lived on ugh the Middle Silurian, Upper Silurian, and 
change. It may ed as a remarkable instance of Darwin's 
of divergence. In the Lower Silurian period this species pumerous 
congeners. But the interval to the Devonian the genus as 4 
became gradually changed, S. rhomboidalis alone retaining ch mire Bir, In 
this comparison, species of Str of 
Montreal, Canada, Sly, 1863. 
