26 SHELL GALLERY. 
breathers. Most of them are provided with shells, and, with the 
exception of the Amphibolidce never possess an operculum. They 
are divisible into two groups or sub-orders, Basommatophora and 
Stylommatophora, characterized by a difference in the position of the 
eyes. The Basommatophora, including the Auriculidm, Amphibolidce, 
Siphonariidx, and Limnceidce, have a single pair of non-retractile 
tentacles, at the base of which are situated the eyes. The Stylom- 
matophora (Land-snails, Slugs, &c), are provided with two pairs of 
retractile tentacles, with the eyes at the summit of the upper pair. 
Over ten thousand species of Pulmonata are known. 
(Basommatophora. ) 
Cases The first group of the aquatic air-breathers, the Auriculidce, 
chiefly inhabit salt or brackish water. The largest forms are tropical 
and found at the mouths of rivers, among the roots and stems of 
mangrove-trees, or in damp woods near the sea. 
Cases The " Limpet-Snails " (Siphonariidce) seem at first sight to be 
out of place anions: the Snails and Slugs, and more nearly allied to 
Three Rows of Teeth of the Radula of Siphouaria. 
c, central ; Z, lateral teeth. 
the Rock-Limpets ; but the character of the tongue (radula) and 
the closed respiratory cavity indicate a close relationship with the 
present group. 
The shells of Siphonaria may be known from Limpets by a slight 
bulging on one side, caused by a radiating groove which interrupts 
the muscle of attachment. They are marine, and are found on rocks 
between tide-marks, chiefly in tropical countries. 
Cases The Limnceidce are only found in fresh water. Most of them 
99-101. occasionally rise to the surface to breathe, where they glide along 
foot uppermost, at times suspending themselves by a glutinous 
thread, after the fashion of a spider. All countries appear to have 
their peculiar species. 
