PHYLOGEXESIS IN CLASSIFICATION. 243 



principles. On account of the relative ease in determination 

 of species in Conchology, the molluscan fossils have always 

 had particular favor with mineralogists and geologists. In 

 any other division of the Animal Kingdom it is impossible to 

 collect, describe, and figure fossil remains in such great abun- 

 dance; and besides, it can be said that the major part of 

 the bibliography in Geology and Paleontology is devoted to 

 shells — not always, it is true, in an ideal manner. If, indeed, 

 the insuf^cient knowledge of living Mollusca is a great cause 

 of frequent errors in the determination of genera, so too the 

 determination of species is at present in an almost chaotic 

 condition. As each author, according to his own views, 

 extends or contracts the limits of species, it happens 

 that one rarely finds in the works of difTerent authors the 

 identical fossils of the same fauna described in the same 

 terms in the definition of characters. A chief cause of this 

 unfortunate state of affairs comes from the vertical range of 

 fossil Mollusca. Very frequently in a series of superimposed 

 beds of different age one meets with a characteristic type of 

 which the specimens from each of the different formations 

 (although presenting minor differences) preserve a special 

 facies throughout. In older works all the mutations of such 

 a series of forms were considered as belonging to one and 

 the same species, while more recently the inclination is con- 

 spicuous either to raise the smallest differences of their kind 

 to the rank of different species, or to distinguish them from 

 one another by application of trinomial names. Mollusca are 

 in major part aquatic. Of these classes, the Tunicates, the 

 Brachiopods, and the Cephalopods, live exclusively in the sea. 

 The greater part of the Bryozoa, the Lamellibranchs, and the 

 Gastropods are found in salt and in fresh water. The class 

 of Gastropoda alone presents representatives living in salt, in 

 brackish, and in fresh waters, and terrestrial species. All the 

 classes capable of preservation appeared in the Lower Silurian 

 (probably all in the Cambrian). The Brachiopods attain the 

 maxim of their development in the Paleozoic age, the Ceph- 

 alopods in the Mesozoic ; the Lamellibranchs and Gastro- 

 pods appear to have continued their differentiation and ex- 

 pansion quite up to the Tertiary or recent period. 



