CEPHALOPODA. 95 



ally, but favourable to the increase of others. The fossil land- 

 snails of St. Helena were supposed by Mr. Darwin to have 

 become finally extinct only in the last century, owing to the 

 destruction of the native woods by the instrumentality of 

 goats and swine. 



Tectibranchiata, — The families typified by Tornatella, 

 Ringicula, and Bulla played a more important part in the 

 secondary and tertiary periods, but their affinities have been 

 seldom understood. The cone-like Acteonina appeared in the 

 carboniferous rocks, and attained a remarkable development 

 in the lias (fig. 27, i°). They were succeeded by the Adeonellce, 

 with a plaited columella, in the cretaceous strata ; and by 

 Volvaria (fig. 28, «) in the eocene. The diminutive Ringiculce 

 of our seas were preceded by large species of the same genus 

 in the tertiaries, and by Cinulia (fig. 27, 9), Globiconcha, and 

 Tylostoma, in the cretaceous strata. The genus Varigera has 

 varices recurring twice in each whorl, like Eulima ; and 

 Pterodonta is winged like Strombus. 



Class IV.— CEPHALOPODA. 



■ These are encephalous Mollusca, with locomotive and 

 prehensile organs radiating from the head (fig. 29, t, h). 

 The animal is divided into a somatal (m, t) and pallial (m, 0) 

 portion. The former is chiefly muscular. It contains the 

 organs of sense, mastication, and deglutition, and, although it 

 supports the organs of prehension and the chief powers of 

 locomotion, it is called the " head," (a, c) whence the name of 

 the class. The pallial division, termed " trunk," or abdomen, 

 consists of a more or less muscular sac or mantle, with a 

 transverse anterior aperture, from which an expiratory siphon 

 or "funnel" (/) projects; and it contains the respiratory, gene- 

 rative, and digestive organs. The branchiae are pinnatifid 

 and concealed. The sexes are distinct. All Cephalopods are 

 oviparous. 



