ZOOLOGY. 577 



Fam. 4. Typical Mussels Mytilacea. 



Occlusor muscles separate, and mantle slit to such an 

 extent, that only the posterior respiratory aperture remains; 

 foot mostly tongue- shaped or coniform, with occasion- 

 ally a byssus.* 



Here belong the fresh-water Mussels and the Mytili 

 proper, of which last many bore into rocks. They 

 typify the Capulidae and Brachiopoda. 



Fam. 5. Snail- Mussels Arcacece. 



Have two separate occlusor muscles and a perfectly 

 separate mantle without respiratory aperture; the foot 

 small, mostly cartilaginous. Here belong the Area?, 

 Aviculae, and Pearl-mussels. 



They typify the Trochidae and Pteropoda. 



Fam. 6. Kracken-Mmsels Ostracea. 



Only one occlusor muscle, mantle entirely open or slit, 

 so that both respiratory apertures are only oblique in- 

 cisions therein ; foot very small, frequently furnished with 

 a byssus. They typify the Buccinidae and Cuttle-fish. 



Fifth Class. 

 Arteriose, Orchitic Animals Snails. 



3495. The Snails likewise divide into two orders, 

 according to the two circles, preindicated by the Ovum- 

 and Sexual system. Their branchiae are either ramulate 

 or pectiniforrn, the sexual parts combined or separated. 



The first kind are still frequently gelatinous in texture, 

 transparent and naked ; their branchiae usually stand out 

 freely as filaments, lamellae, orranmles upon the back, or 

 lie simply as a vascular network within the mantle. All 

 are androgynous. They therefore obviously repeat the 

 Protozoa or Mucus- animals. 



The second are invariably covered by the shell and by 

 a mantle, within the cavity of which the branchiae he con- 

 cealed as one or two comb-like bodies. Tentacula and 

 eyes, which are occasionally wanting in the preced- 

 ing order, are here universally present ; the sexes se- 

 parate. 



As in the Snails, the male parts make their appearance 



37 



