THE NAUTILUS. 31 



ill the limits of the region. Two terrestrial forms Patula strigosa 

 Gould, and P. alternata Say, often still retain in the fossil state the 

 red surface markings. Several similar cases other than those here 

 referred to, might also be mentioned, but it is scarcely necessary in 

 the present connection. 



In its broadest sense, the term fossil is applicable to all naturally 

 buried organic remains ; whether recently entombed, or having 

 been inclosed for countless ages in the earth's sti'ata. In the general 

 process of petrifaction of organic structures the animal matter is 

 quickly destroyed and only the hard parts escape obliteration. 

 When the proportion of organic to inorganic material is large there 

 is usually a more or less complete effacement of all indications of 

 life ; but when the mineral constituents predominate, as in the shells 

 •of mollusks, brachiopods, and the tests of echinoderms, etc., these 

 parts are often preserved intact, with simply a loss of animal matter. 

 It frequently happens, however, that the calcareous shell is gradually 

 and completely replaced by some other mineral substance, as iron or 

 silicon, yet preserving perfectly the form and ornamentation. As 

 might be expected under the circumstances the original coloration 

 of fossil shells is very rarely retained ; and the few instances noted 

 are therefore of particular interest as revealing certain ])hases of 

 molluscan life that existed in ages gone by. In the later geological 

 ■deposits the retainment of some trace of coloration in shells is of 

 ■course very much more probable than in the earlier rocks. 



THE ISAAC LEA CHAPTER OF THE AGASSIZ ASSOCIATION. 



BY I)R. M. L. LEACH. 



Referring to the organization of the American Association of 

 Conchologists, notices of which have appeared in recent numbers 

 of the Nautilus, it may not be out of place to mention that a similar 

 society, having the same objects in view, is already in the third year 

 of its existence. The Isaac Lea Chapter of the Agassiz Association, 

 is made up of members widely scattered over the country, to whom 

 the study of conchology is of special interest. There has never 

 been a meeting of the Chapter, but the voting and all society 

 business is done by correspondence. No fee is required for admis- 

 sion, and there are no assessments or dues. The members are 



