Ixvi PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



the later formations, when the genera of Ammonitoida and Nautiloida 

 had become scarce or disappeared for ever, was interpreted only as a 

 continuance of the same class under new and minuter forms. Analogy 

 was mistaken for affinity ; and substitution of one group for one totally 

 and organically different, although in the mere form of test not 

 dissimilar, was mistaken for succession and representation within the 

 sphere of one type. But the discovery of Dujardin led the way to 

 an entirely new interpretation of the value of the Rhizopoda, and a 

 new view of the part they play in time. Proving, from good evidence, 

 to be among the lowest of animal forms, to be in fact Protozoa like 

 Amoeba, but differing from both Proteus and the animal element 

 of the sponge by their investment with a hard and symmetrically 

 arranged (generally in spiral symmetry) exo-skeleton, it is most inte- 

 resting to note that their advent and maximum development have been, 

 not during the apparent dawn of life, but amid the later epochs, and 

 chiefly during those ages which many palaeontologists regard as espe- 

 cially characterized by the highest forms of the animal kingdom. 

 Indeed, so far as we know at present, the whole great group of 

 Protozoa — the group that stands as it were at the very base and con- 

 stitutes the rudiments of the animal series — is as characteristic of the 

 tertiary section of time as the Vertebrata themselves are. A com- 

 parable phasnomenon is becoming rapidly manifest in the moUuscan 

 subkingdom, now vastly increased by the accession of the Polyzoa to 

 its ranks. These curious, lowly-organized, zoophytoid mollusca, 

 instead of being the first of their type to appear, were preceded by 

 members of all the higher orders of it, and do not become of much 

 chronological value until the testaceous forms of the highest class of 

 Mollusks occur, few and far between, and lose their strength and 

 their importance. 



The exquisite symmetrj^ and regularity of conformation of the 

 shells of most recent and fossil Rhizopoda were the chief sources of 

 the errors that prevailed so long about their nature and zoological 

 position. The trvie explanation of their structure appears to me to 

 be that given in detail by our fellow-member Dr. Carpenter, to the 

 effect that the entire mass, however symmetrical or regular, repre- 

 sents the products by successive gemmation originating from a 

 single ovum. It matters little whether we regard each 'joint' or 

 cell of a Nummulite as representing an indi%idual or a zooid, pro- 

 vided we regard it as an element of the same essential nature with 

 each polype of a polypidom, each cell-animal of a polyzoon, or indi- 

 vidual of a Botryllus. The value of the regularity of the whole is 

 not invalidated, because that whole is a compound and not a unity, 

 and our faith in the specific value of the fossil, and its consequent 

 geological importance, may be as strongly based on the constancy of 

 characters whose diagnosis is drawn from the features presented by a 

 congeries of individuals as from those presented by a single being. 

 I make this remark, because the only serious objection that I can take 

 to the views of M. d'Archiac touching the nature of the Nummulite 

 concerns this fundamental point. When he states as an argument 

 against its compound nature, that, if each of the cells were the proper 



