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 branchiae 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 branchifera, and corresponds 

 with De Blainville's sub-class dioica. The branchiae 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 : 1st, Siphonostomata, corresponding with De Blain- 

 ville's order sip/fonobranc/iiata, 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, Holostomata, 

 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 



