100 GASTROPODA. 



attached for its whole length (save in Valvata). The 

 nerve-ganglia distinct and concentrated round oesophagus ; 

 pedal commissures rare. Genital aperture distinct, 

 dioecious with rare exceptions. 



To this order belong by far the greater number of 

 Prosobranchia, all, indeed, of those the development of 

 which is dealt with here except the forms named above. 

 Jftuthina, Murex, Bnwinnm, Pttr/mi'<(, N>!*a, Fulf/ur, 

 Fuswij Fasciolaria, Strombut, Rostellaria^ Crepidula, 

 C'dyptraea, Vrrmetus, Bythinia, Pa/udit/a, Tliii<:, Stili- 

 fer, Entoconcha, etc. 



Sub-order 3. Heteropoda (Nucleobranchia). The char- 

 acter of the nervous system, the position of the gill, 

 ventricle and auricle the same as in the Monotocardia. 

 Foot developed into a fin. 



Oxygyrus, Atlanta, Pterotmekva, Carinari<i, 



II. OPISTHOBRANCHIA. 



The gill and auricle generally behind the ventricle (except in 

 Actaeon). Pleuroviseeral commissures rarely crossed 

 (Actaeonidae). Hermaphrodite, marine. 



Sub-order 1 . Tectibranchia. Shell generally present, often 

 much reduced and internal,, wanting in Rmicina and 

 Pleurobramhea ; with mantle -cavity containing a cteni- 

 dium. 



Actaeon, Bulla, Acera, GaxteropteroH, PltilnK*, Aplysify 

 Pleurobranchus, Pleurobrancliea, Umbrella. 



Sub-order 2. Nudibranchia. Without shell in adult stage ; 

 mantle, ctenidium and osphradium wanting. 



Trltonia, Doris, Chromodoris, Polycera, Tert/ipe*, Elyxia, 

 Aeolix, Doto, 



III. PTEROPODA. 



Pelagic Gastropods in which the head is much reduced, and 

 the foot is developed like a fin ; now generally classed 

 with the Opisthobranchia. 



Sub-order 1. Thecosomata. With calcareous or carti- 

 laginous shell, with mantle and mantle-cavity. 



Spirialis, Limacina, Tiedeniannia, Cymbtdui, Gvolima f 

 Hyalocylix, Styliola, C/eodora, Creseis. 



