248 Proceedings of the American Association. 
earliest to the most recent, present themselves as living forms in 
the present ocean. 
Among the Gasteropoda, we find several generic forms which 
thus far have not been recognized in the higher strata. Among 
these are Maclurea, Scalites, Bucania, Cystolites and others, 
which are common, and some of them abundant forms in the 
lower period. The genus Murchisonia likewise appears, with 
us, to belong almost exclusively to the lower Silurian period, few 
or no species having been recognized in a higher position. The 
genus Pleurotomaria, on the other hand, appears throughout the 
Paleozoic strata. 
e genus Acroculia does not appear in the lower Silurian 
strata, and is but faintly represented in the lower stages of the 
upper Silurian, while in the higher strata of the same division, it 
is more abundant than any other univalve. It is likewise con- 
tinued into the higher or Devonian strata, being more abundant 
near the commencement of this period, and dying out towards 
the termination of the same. 
We recognize among the Cephalopoda the genera G'onioceras, 
Hudoceras and Oncoceras, which possess peculiar characters un- 
known in shells of this order in the subsequent stages of the sys- 
tem, while Ormoceras seems to mark even a more limited period 
of the lower stage of the system. The genus Litnites, which 
marks the lower divisions of the system, is scarcely represented 
in the second and third stages, even by any analogous form, while 
Cyrtoceras, and afterwards Goniatites, follow in the 
period in much greater numbers, both of species and individuals, 
than we find of the former genus in the lower term of the system. 
Almost all the well established genera of American Trilobites, 
appear in the lower term of the system. The genera Isotelus 
and Illenus, with Trinuecleus, are the prevailing and typical 
forms, which are unknown in any higher position. The genus 
Ceraurus seems likewise typical of strata of the lower Silurian 
period, as well as the genus Olenus, of which we have some im- 
perfect specimens. 
The genera Calymene, Phacops, Asaphus, Platynotus or Li- 
chas, are known in the subsequent stages of the system. The 
genus Calymene is no where else so abundant as in the lower 
term of the system, though scarcely less so in the middie, or lower 
stage of the upper Silurian, while it is scarcely known above the 
base of the Devonian system. The genus Phacops, which is 
but poorly represented in the lower term of the system, is more 
abundant in the middle, and one species, the ?. macrophthalma; 
is rare in the Silurian period, but becomes abundant in t 
vonian. ‘The genus Asaphus appears near the base of the upper 
ilurian, but is unknown to us in the Devonian. ‘The genus L’- 
chas, (Platynotus, Conrad,) is represented by a single species in 
