280 L. Agassiz on the Primitive Diversity | 
the preceding computations, which 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 formations, on account of the 
softness of their body, which will scarcely allow of preserva- 
tion to a greater degree than Medusz. But a few instances 
in which impressions of these animals have been found jus- 
tifies the assertion that they existed as well in former periods 
asnow. The impressions of Meduse found in the lithographic 
limestone of Solenhofen, which are preserved in the Museum 
of Carlsruhe, not only carry back the existence of this 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 of Meduse similar to the Campanulariz and Ser- 
tulariz of the present day, which are now known to be no 
Polyps, but one of the alternate generations of Meduse. And 
as for the worms, we find in each geological formation, from 
the oldest to the most recent, fossil Serpulz, or similar solid 
cases of worms in as large numbers as we find these animais 
anywhere at the present day. And where the existence of 
Serpule is established by such unquestionable evidence as 
that of their calcareous cases, are we not justified in the in- 
ference that those entirely naked worms, which are found 
everywhere existing with Serpule, had also their corre- 
sponding representatives during former geological periods ? 
With the class of Crustacea the difficulty in the comparison 
is already less ; for, in the tertiary beds of Sheppy there have 
been found a variety of lobsters, shrimps, and crabs, which | 
would favourably compare with the crab fauna of any limited 
shore in the present day ; and I doubt very much whether 
such a variety of Crustacea could be collected anywhere on 
a shore of equal extent to that of the white chalk of Sussex, 
as Dr Mantel has uncovered in the vicinity of Lewes. For 
a comparison of the Crustacea of the oolitic period, I would 
only refer the sceptic to the monograph of the Crustacea of 
Solenhofen by Count Miinster, who has figured from that 
single locality more species than are known in the whole 
