L. Agassiz on Animals in Geological Times. 315 
oolitic period, or any of the subdivisions of that formation, sur- 
pass the number of species of that class which may be gathered 
around the coast of entire continents in the present day. The 
diversity of forms of these animals comparing them with those 
of the cretaceous periods, is equally great, though the Crinoids be- 
gin to diminish in number. But the variety of Spatangoids and 
Clypeastroids which come into play, compensate largely for the 
diminution of the family of Crinoids. 
The type of Articulata may seem, in the present condition of 
our knowledge, to form an unanswerable objection to the broad 
statement [ have made above, for the- hundred thousands of in- 
sects which are known in the present creation will hardly allow 
a comparison with the fossils. But let us examine upon the prin- 
ciples by which we have been guided in the preceding computa- 
tions, what is the true state of things respecting the occurrence 
of Articulata in former geological periods. We can, of course, 
hardly expect to find worms well preserved in geological forma- 
tions, on account of the softness of their body, which will 
scarcely allow of preservation to a greater degree than Meduse. 
But a few instances in which impressions of these animals have 
been found justifies the assertion that they existed as well in for- 
mer periods as now. ‘The impressions of Meduse found in the 
class to the Jurassic period, but justify the question whether a 
large number of the fossil polypi from older periods, which have 
been described as belonging to that class, are not in reality nurses 
Medusz similar to the Campanularie, and Sertularie of the 
Present day ; and I doubt very much whether such a variety of 
Crustacea could be collected any where on a shore of equal extent 
'0 that of the white chalk of Sussex, as Dr. Mandell has uneov- 
in the vicinity of Lewes. For a comparison 0 
'acea of the oolitic period, I would only refer the skeptic to the 
