ORDER PROSOBRANCHIATA. 3J1 



terous Mollusca by the possession of an operculum. They have 

 an ample spiral shell, and only possess two tentacles, with the 

 eyes on little tubercles at the outside of their bases. In the 

 genus Cyclostoma, of which one species is found on chalky soils 

 in this country, the loot is divided beneath by a longitudinal 

 groove, and each half is used alternately in walking. Most of the 

 species are inhabitants of warm climates, and they resemble the 

 Snails in their habits. 



ORDER II. PROSOBRANCHIATA. 



984. The Prosobranchiata, which are nearly all marine, con- 

 stitute by far the largest and most important Order of the Gas- 

 teropoda. They are all furnished with an external shell, which 

 is usually of a spiral form, with the last turn of the spire large 

 enough to receive the entire animal within its cavity ; the 

 branchiae, which are of a pectinated or feathered structure, are 

 almost always placed in an arched chamber formed by the man- 

 tle, and situated above the neck, and in front of the heart, the 

 auricle of which is placed in advance of the ventricle. The 

 species in which these characters are exhibited in the greatest 

 perfection form the great section of the Pectinibranchiata, which 

 includes by far the greater portion of the Order. The remainder, 

 which may be regarded as aberrant forms, constitute two smaller 

 sections, of which the Cyclobranchiata have the plume-like 

 branchiae situated at the margin of the body, between the man- 

 tle and the foot ; whilst the Cirrhobranchiata are distinguished 

 by having the head surrounded by numerous cirrhiform organs, 

 which have been regarded as branchiae by many authors, al- 

 though their function appears still to be rather problematical. 

 As the families belonging to this Order are exceedingly numer- 

 ous, we shall only refer in detail to some of the most important 

 of them. 



