18 CYCLOSTOMID^E. 



The Cyclostomida differ from the true Pneumono- 

 branchiata, with which they have been generally 

 previously confounded, in the mantle being free 

 from the back of the neck, leaving the pulmonary 

 cavity open, and in the animals being unisexual and 

 provided with a distinct well-developed spiral oper- 

 culum. 



M. Moquin Tandon observes that in the front of 

 the pulmonary cavity of Cyclostoma elegans, near the 

 collar, the pulmonary vessels are closer together, pro- 

 ducing some parallel slightly flexuous ridges, which 

 appear to be rudimentary gills. (See pp. 74. 166.) 



These animals, when they walk, apply their pro- 

 boscis to the ground to assist them in progression. 



1. CYCLOSTOMA. (Circle Shell.) 



Shell ovate-spiral ; mouth simple, united all round ; 

 operculum of a few flat whorls, with a simple 

 shelly internal coat ; the foot divided into two 

 parts by a longitudinal central groove. (See 

 fig. 4. p. 17. Operculum rather ovate, double- 

 edged, internally concamerated, with transverse 

 concentric and septa. Shelly whorls 4 or 5, 

 gradually increasing. Nucleus excentric. 



The foot is formed of two longitudinal portions : 

 as the animal walks, the portion on one side is first 

 advanced, while the animal holds on by the other ; 

 and then holds on with the advanced portion as the 

 other side is gradually advanced before it. (See 

 p. 17. f. 4. Rossmasler, Icon. i. 89. t, 2. f. 80. 82.) 



This is the case with Cyclostoma lineolata and the 



