ENCEPHALA. 



79 



an A&pergillum. Specimens have been obtained in which the 

 whole interior of the valves and tube had been excessively 

 thickened towards the close of life by successive layers of 

 shell. 



Class III— ENCEPHALA. 



About three-fourths of the Mollusca are " encephalous," or 

 have a distinct head, commonly with eyes and tentacles, and 

 the mouth has a peculiar and complex preparatory organ of 

 digestion. 



The mantle, properly so called, is the free fold or folds of 

 the skin, produced usually from the dorsal surface, and is in 

 functional relation with the breathing organ and the shell. 

 By the preservation of the latter we learn that the encephalous 

 grade of molluscous organization dates from the deposition of 

 the old Silurian beds now forming the Llandeilo rocks. 



Sub-Class.— PTEKOPOD A. 



The Pteropoda are so called on account of the resemblance 

 of their principal organs of C 



motion (fig. 21, C) to a pair of 

 wings, both as to form and in 

 their mode of action on the 

 surrounding medium. They 

 are small marine floating 

 species, and might leave evi- 

 dence of their existence in 

 deposits of the deepest ocean. 

 The greatest extremes of form 

 are presented in this order, — 

 some species of Hyalcea (fig. 

 22) being almost globular ; others, as certain Gleodorce being 

 very long and slender. The shell is always characterised by 



Fig. 21. 

 Hyalcea tridentata, magn. 



