124 EOCENE MOLLUSCA. 
arborescent or fasciculated gills, which are not enclosed in a special cavity, but are 
more or less completely exposed, either on the back or on the sides towards the hinder 
part of the body. The reproductive apparatus is hermaphrodite (not in the sense used 
by De Blainville, but meaning that the sexes are united in the same individual), and 
the shell is either wanting or is merely rudimentary in the adult state. In the second 
division, the abdomen, which is developed proportionately with the cephalic and 
locomotive masses, is always protected by a shell, generally of sufficient size to contain 
the whole animal. ‘The mantle forms over the cervical region a vaulted chamber, more 
or less capacious, in which the branchiz are lodged and the excretory orifices are placed. 
The reproductive organs, male and female, are borne by different individuals. This 
division comprises Cuvier’s four remaining orders of the dranchifera, and corresponds 
with De Blainville’s sub-class dioica. The branchize are composed of simple and par- 
allel plates, arranged, somewhat like the teeth of a comb, along a vascular stem, and, 
for the most part, are placed obliquely across the back, or are attached to the right 
side of the neck. 
In some genera in this order, the edge of the mantle is prolonged into a canal or 
siphon, which can be extended at pleasure, so as to permit the free passage of water 
into the branchial chamber, while the animal itself remains within the shell ; and where 
this siphon exists, the front of the aperture of the shell presents a notch, or is produced 
into a channel in which the siphon rests. In other genera the respiratory siphon is 
altogether wanting, or its place is supplied by a lobe developed from the neck, and in 
these genera the aperture is without the anterior notch or channel. Sometimes a 
posterior tube exists with a corresponding notch or canal in the shell; but the function 
of this posterior tube is simply to provide for the more easy efflux of water or the 
ejection of the anal excretions from the branchial chamber. The head of the 
prosobranchiate gasteropod is provided with tentacles, which serve as organs of touch, 
and probably of smell, and with a proboscis which in some genera is retractile or 
exsertile. The eyes, with which organs all are endowed, are generally placed either 
at the bases, or on the extremities, or the sides of the tentacles; but in some genera 
they are carried on pedicels specially appropriated for them. 
The presence or absence of the respiratory siphon has been used for the subdivision 
of the present order into two sections: Ist, Siphonosfomata, corresponding with De Blain- 
ville’s order siphonobranchiata, and comprising such of the prosobranchiata whose pro- 
boscis is retractile, and the margin of whose mantle is prolonged into a siphon, and whose 
shell is, consequently, notched or produced into a channel in front: and 2d, //olostomata, 
consisting of those in which the proboscis is not retractile, and the animal not being 
provided with a respiratory siphon, the aperture of the shell is entire. The genera 
comprised in the first section are all zoophagous, and are inhabitants of the sea or of 
brackish water; those in the 2d section are, for the most part, phytophagous; the 
greater number live in salt or brackish water; some, however, are inhabitants of fresh 
