356 M. L. Agassiz on the Primitive Diversity and 



set of beds known under the name of Niagara limestone, equals 

 the whole number of Echinoderms found around all the coast of 

 the United States. The Crinoids, Echini, and Star-fishes of the 

 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 

 begin 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 I have made above, for the hundred thousands of 

 insects which are known in the present creation will hardly allow 

 a comparison with the fossils. But let us examine, upon the 

 principles by which we have been guided in the preceding com- 

 putations, what is the true state of things respecting the occur- 

 rence 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 preservation to a greater degree than Medusae. 

 But a few instances in which impressions of these animals have 

 been found justifies the assertion that they existed as well in 

 former periods as now. The impressions of Medusse 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 w^hether 

 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 Medusse similar to the Campanularice and Sertularia of 

 the present day, which are now known to be no Polyps, but one 

 of the alternate generations of Medusse. And as for the worms, 

 we find in each geological formation, from the oldest to the most 

 recent, fossil Serpula, or similar solid cases of worms in as large 

 numbers as we find these animals anywhere at the present day. 

 And where the existence of Serpula is established by such un- 

 questionable evidence as that of their calcareous cases, are we not 

 justified in the inference that those entirely naked worms which 

 are found everywhere existing with Serpula, had also their cor- 

 responding representatives during former geological periods ? 



With the class of Crustacea the difiiculty in the comparison is 

 already less ; for in the tertiary beds of Slieppey 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 



