140 



Order iii.— Opisthobranchiata. . 



(Tectibranchiata) (Xuilibraiicliiata) 



Tornatel]id:>} Dorldse 



BiilliJaa TritoniadsB 



Aplysiad* CEolidas 



Pleurobranchidae Phyllirhoidaa 



, Phyllidiadse Elysiadae 



Order iv.— Nuclkobeanchiata. 

 Firolidre *Atlantidse 



The common Helix or garden snail may be regarded as the type of the 

 second class of Molluscs : they derive their name from the muscular disc on the 

 heUy, by the successive expansion and contraction of which they are enabled to 

 efifect their creeping motion. They inhabit salt or fresh water or the land, and 

 their breathing organs are modified to suit these various spheres of existence ; 

 the knowledge of their affinities, therefore, often enables the geologist to 

 determine with tolerable certainty whether a formation or series of beds is marine 

 estuarine, fluviatile, or lacustrine. Their heads are furnished with distinct 

 tentacles and perfectly-formed eyes, with a simple apparatus for hearing. They 

 have a less-developed nervous system than the Cephalopoda, and may be looked 

 upon as presenting the type of molluscous organization, with the least alliance 

 to the Vertebrate or Articulate sub-kingdoms. The shell, with which these 

 animals are usually protected, is univalve and variously-shaped, its typical form, 

 however, being conical and spiral ; the aperture is often closed by a Ud 

 called an operculum, which the animal brings behind it, when it withdraws 

 into the shell : the variations of tliis organ are of some use in subordinate 

 classification. 



This large class is divided into orders according to the character and 

 position of the breathing organs. In treating of Silurian palaeontology, we have 

 to deal with the first and fourth orders only ; and we shall therefore dismiss 

 the others with a very brief notice. 



The PULMONIFERA (snails, slugs, &c.) inhabit either the land or 

 fresh water ; they breathe air by means of a vascular sac at the side of the 

 neck, forming a sort of luwj ; they are generally furnished with a spiral shell 

 sometimes covered with an operculum, sometimes not. The earliest trace of 

 this order was met with in Nova Scotia by Sir C. Lyell, who found a Pupa 

 of the family ffelicida in an erect fossil tree of the Coal Measures ; the 

 Upper Purbeck beds contain shells of the family Limnceidce ; but these Molluscs 

 are of rare occurrence before the Tertiary formations. 



In the Opisthobranchiata, whose gills covered or bare are situated 

 behind the heart, the shell is rudimentary or wanting : one member of the first 

 family Adeonina appears in the Carboniferous rocks, but they seem to have 

 fiourished most during the deposition of the Secondary and Tertiary formations. 



