OR OSSEOUS SYSTEM. 51 



and many others, the interior surface of the shell has 

 experienced no change by being long in contact with the 

 living soft parts of the contained animal ; the grooves, 

 the striae, the prominences, and even the colouring mat- 

 ters which were upon the outer exposed surface in the 

 young condition of the shell, still preserve in the adult 

 state, their primitive appearance and integrity throughout 

 the whole interior of the revolutions. These have grown 

 by the revolution of the upper lip around the columella ; 

 but many others add new layers also to the inferior or 

 columellar lip during their growth, thus covering over 

 the superficial external* layers on the left side of the 

 revolutions, and generally presenting strong parietes and 

 a thick and solid columella, as in the strombus gig as. In 

 many forms, however, as in the cones and olives, where the 

 widely expanded upper lip sufficiently covers and protects 

 the smaller revolutions, the total weight of the shell is 

 diminished without weakening its exposed part by carrying 

 forward the calcareous matter from the inner first formed 

 concealed revolutions, and depositing it upon the exterior 

 surface of the last or outer turn of the shell which alone 

 is exposed to danger from external causes. The vertical, 

 zig-zag, parallel fibres composing the thick parietes of 

 these shells is distinctly preserved in their fossilized re- 

 mains. The hybernating gasteropods, as the snails, which 

 want an operculum, close up the aperture of their shell, 

 when they retire to their winter slumber, with a thick 

 deposit of calcareous matter, called an cpiphragma, which 

 is not connected with the muscular foot, like the true 

 operculum of other shells, but only with the aperture of 

 the cone. The operculum is a permanent part attached 

 to the contained animal, and the epiphragrna is a de- 

 ciduous part attached only to the orifice of the shell. 



XVII. Pteropoda. As the pteropodous animals are not 

 provided with a muscular foot to creep upon a solid sur- 

 face, but are all organized to swim freely through the 

 sea by means of muscular expansions like fins, they are 

 never encumbered with a massive or heavy skeleton. 

 Their skeleton, when present, is generally external, extra- 

 vascular, thin, pellucid, horny, or vitreous ; it is univalve, 

 unilocular. of various forms, generally without a spiral twist, 



K 2 



