MOLLUSCA. 23 



ORDER III. TRACHELIPODA. 



Body spirally convolute in its posterior part, separated from the 

 foot, and always enveloped by a shell ; the foot free, flattened, 

 attached to the inferior base of the neck, or the anterior part 

 of the body, and forming an instrument of locomotion ; shell 

 spiral, enveloping. 



The animals of this very extensive order are arranged in the following manner : 

 SECT ION I Trachelipoda with protruding syphon, and respiring water only, which 

 arrives at the branchiae by this syphon. They are all marine ; feed upon other 

 animals ; are destitute of maxilla? ; and furnished with a retractile proboscis. 

 Shell spiral, enveloping, with the aperture canaliculate, notched or effuse at its base. 



a. Shell without canal, but having the base of its aperture notched or effuse, and its 

 spiral turns broad, compressed, and rolled up so that the last almost entirely covers 

 the others. Involute/,. 



b. No canal at the base of the aperture, but a subdorsal notch, and folds upon the 

 columella. Columellaria. 



c. Shell with a short canal, ascending posteriorly, or an oblique notch in the form 

 of a short canal at the base of its aperture, this canal having a direction toward 

 the back. Purpurifera. 



d. Shell with a canal of greater or less extent at the base of its aperture, and of 

 which the right margin changes its form as the animal advances in age, and with 

 a sinus beneath. Alata. 



e. Shell with a canal of greater or less extent at the base of its aperture, and of 

 which the right margin does not change its form as the animal advances in age. 

 Canalifera. 



SECTION II Trachelipoda without protruding syphon, and respiring in general 

 by a hole ; the greater number phytiphagous and furnished with maxillae ; shell 

 with the aperture entire, not having at its base either a subascending dorsal notch 

 or a canal. 



* Trachelipoda respiring only water ; branchiae protruding in the form of filaments 

 laminae or tufts in the branchial cavity ; shell often nacreous, and often also with 

 protuberances at its surface. 



a. Shell marine, the last margin of the aperture not septiforrn. 

 f No folds on the columella. 



a. The margins of the apertuie disunited. Turlinacca. 



b. The margins of the aperture circularly continuous. Scalaria. 



"!"} Aperture without particular widening folds on the columella. Pticacea. 



f-ff Shell not floating, having the aperture very wide ; no columella. Macros- 

 toma. 



j"|"|"f- Shell floating at the surface of the water. Janihince. 



b. Shell fluviatile or marine, the left margin septiform. Neritacea. 



c. Shell fluviatile, operculate, the left margin not septiform. 

 f Shell with united margins. Peristomia. 



tt she11 with disunited margins. Melaniacea. 



** Trachelipoda respiring only air ; shell spiral, without canal or notch, not dis- 

 tinctly nacreous. 



a. Those which live in the water, but which come to the surface to respire air ; shell 

 with the edges of the aperture never reflected. Lymnacea* 



b. Those which do not live in water. Colimacea. 



