Vol. XI] 



BERRY— FOSSIL CHITONS 421 



Placiphorella velata 



Ischnochiton fallax 



Ischnochiton magdalenensis 

 " cooperi 



Upper San Pedro Mopalia acuta 



Ischnochiton acrior 



" pectimdatus 



" sancta?monica> 



Here, too, later work will no doubt bring about appreciable 

 modification. One feels inclined to prophesy that Katharina 

 tunicata will be found to occur in the Lower San Pedro as well 

 as in the Pliocene, while it also seems reasonable that a large 

 proportion of the species just now apparently characteristic 

 of the Lower Pleistocene will eventually be discovered in the 

 Pliocene as well. In addition to the forms specifically char- 

 acteristic as noted, the Upper San Pedro horizons seem gen- 

 erally to be marked by an extraordinarily abundant development 

 of Callistochiton as compared with individuals of other genera. 



Parts of Animal Preserved as Fossils 



In chitons, as probably in every division of animals, all 

 normal structures possess some special element to contribute 

 toward a properly phylogenetic classification of the group. In 

 such a classification it is no more proper to neglect a certain 

 organ or group of organs than it is to leave out of consideration 

 some difficult species or genus. Probably no one would dispute 

 this, theoretically at least, but as a matter of actual practice, 

 and with no more data to work with than we have at present, 

 it is quite impossible, as in almost all other groups, even rea- 

 sonably to approach so ambitious an ideal. For the time being, 

 we must perforce be content with putting our reliance upon 

 those structures which by reason of hardness, like the shell, or 

 of adaptability to mounting, like the radula and girdle scales, 

 are readily susceptible of preservation. 



In the case of fossils the very nature of things limits us to 

 consideration of the shell, or, rather, to the shelly plates, eight 

 in number, into which the chiton shell is divided. In life these 

 are regularly meristic in position and are held together by the 

 stout chitinous or leathery girdle, itself variously beset, both 



