LIMACINAD.E. 85 



1839) ; to Eschricht's " Anatomische Untersuch- 

 ungen iiber die Clione borealis" (Copenh., 1838) ; 

 and to Rang and Souleyet's " Histoire Naturelle 

 des Pteropodes" (Paris, 1852) ; we are indebted 

 for nearly all our knowledge of the structure and 

 affinities of these little animals. 



PTEKOPODA. 



Molluscous animals provided with a head, and 

 with two muscular fins, by which they swim at 

 large in the sea ; sometimes partly enclosed in a 

 glassy shell. 



Animal furnished with a shell : 



Shell symmetrical, two-valved . , Hyaleada. 

 Shell spiral, with an operculum. . Limacinadce. 



Animal destitute of a shell . . . Clionidtf. 



FAMILY I. HYALEAD^E. 



Animal furnished with an external shell ; head 

 indistinct ; foot and tentacles rudimentary ; shell 

 symmetrical, globose, formed of two plates. 



Hyalea (Lamk.). Glass-shell. Shell slit at the 

 sides and front, three-pointed behind. 



H. trispinosa. Fig. 124 ; n.s. 



FAMILY II. LIMACINADCE. 



Animal with a spiral, snail-like, but sinistral 

 shell ; head indistinct ; fins attached to the sides 

 of the mouth, united ventrally by a lobe. 



Spirialis (Souleyet). Shell minute, furnished 

 with a thin, glassy, crescent-shaped operculum, 



