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GEOLOGICAL CONTEMPORANEITY 299 



groups which have been mentioned, and many 

 besides, exhibit no sign of progressive modification, 

 there are others, co-existing with them, under the 

 same conditions, in which more or less distinct 

 indications of such a process seems to be traceable. 

 Among such indications I may remind you of the 

 predominance of Holostome Gasteropoda in the 

 older rocks as compared with that of Siphonostone 

 Gasteropoda in the later. A case less open to the 

 objection of negative evidence, however, is that 

 afforded by the Tetrabranchiate Cephalopoda, the 

 forms of the shells and of the septal sutures 

 exhibiting a certain increase of complexity in the 

 newer genera. Here, however, one is met at once 

 with the occurrence of Orthoceras and Baculites at 

 the two ends of the series, and of the fact that 

 one of the simplest genera. Nautilus^ is that 

 which now exists. 



The Crinoidea, in the abundance of stalked 

 forms in the ancient formations as compared with 

 their present rarity, seem to present us with a 

 fair case of modification from a more embryonic 

 towards a less embryonic condition. But then, on 

 careful consideration of the facts, the objection 

 arises that the stalk, caljrx, and arms of the palae- 

 ozoic Crinoid are exceedingly different from the 

 corresponding organs of a larval Comahda ; and it 

 might with perfect justice be argued that Actino- 

 crinus and Eucalyptccrinus, for example, depart to 

 the full as widely, in one direction, from the stalked 



