18 
CYCLOSTOMID^. 
The CyclostomidcB diflfer 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. Eossmasler, Icon, i. 89. t. 2. f. 80. 82.) 
This is the case with Cyclostoma lineolata and the 
j- 
