POLYGYRA ALBOLABRIS AND LIMAX MAXIMUS 245 



The prismatic part of the shell is apparently formed by the 

 deposition of calcareous matter in prismatic, generally hexagonal, 

 cells, which are themselves formed by the successive secretions of 

 fenestrated laminae, deposited by the margin of the mantle (pi. 

 2, fig. 12 and 15). 



The laminar part is apparently composed of numerous thin 

 laminae varying in thickness. In many thin sections v^hich I 

 have examined under the miscroscope I have seen no trace of the 

 prismatic structure in this part of the shell. The nacreous layer 

 is very thin, forming the inner stratum of the shell, and is of a 

 somewhat pearly appearance. This layer is a part of the 

 laminar stratum, though it seems to be discernible, and in the 

 broken edges of the shell is always distinct and is distinguished 

 by its lighter color. - 



Till the shell reaches maturity the margin is very thin; but 

 when mature the margin becomes greatly thickened and reflected, 

 forming a broad peristome (pi. 1, fig. 3, 4-7, 12). It is apparently 

 formed by the part of the mantle depositing the prismatic layers, 

 as nearly its entire thickness is of prismatic structure (pi. 2^ 

 fig. 13, 14). 



If the margin of the shell is broken, it can be repaired by the 

 animal; but, if any other part of the shell is broken, complete 

 renewal is impossible, as the prismatic and cuticular layers of the 

 shell are deposited only by the thickened border of the mantle. 

 A mucoid substance in such cases is excreted and becomes calci- 

 fied, taking the place of the missing shell; but any considerable 

 breakage of the shell, except at the margin, is liable to be fatal 

 to the animal. In collecting specimens, sometimes by accident 

 the shells were broken; in nearly every case the animals died 

 within a few days. 



External features 



The ventral part of the animal is expanded into a locomotor 

 organ or foot, which is a thick, vermiform body with a ribbon-like 

 ventral disk. The head is situated at the anterior extremity, 

 and is obtuse; from it protrude two pair of tentacles, the upper 



