250 NICOLAUS STENO 



greater length, considering first shells taken from the sea, and 

 then those which are dug from mountains. 



Shells of every kind which at one time had a living creature 

 enclosed in them, reveal to our perceptions the following 

 characteristics : 



1. The entire shells are themselves resolved into subdivi- 

 sions, the subdivisions, again, are divided into filaments, and 

 these filaments are reduced to two kinds differing from each 

 other in color, composition, and place. 



2. In the subdivisions of the shell the upper and lower sur- 

 faces are nothing but the ends of filaments, while the surface 

 of the edge is the sides of those same filaments located in the 

 edge of the subdivision. 



3. The inner surface of the shell itself is identical with the 

 inner surface of the inmost or largest subdivision, while the 

 outer surface is composed of the outer surface of the smallest 

 subdivision, and of the surface of all the edges of the interme- 

 diate subdivisions. 



Regarding the manner in which shells on animals are 

 formed, the following points can be clearly shown: 



1. That the substance of the filaments is like the perspira- 

 tion of animals, in that it is the fluid exuded through the outer 

 surface of the animal. 



2. That the form of the filaments can be produced in two 

 P. S4. ways : either in the animal's very pores, through which they 



are exuded, or the surface of the growing animal, having be- 

 come larger than the surface of the subdivisions already hard- 

 ened, separates from it, and so partly draws the viscous fluid 

 contained between the two surfaces into filaments (a process 

 which is common to viscous fluids), and partly adds to it by the 

 exudation of fresh fluid, because no other substance can enter 

 between the two surfaces. 



3. That the difference of the filaments depends upon a dif- 

 ference of the pores by which the surface of the animal is per- 

 forated, and upon a difference of the substance which is exuded 

 through the pores ; for animals of this kind po-ssess a twofold 

 substance in their surface, of which the one is harder, the other 



