558 HISTORY OF SYSTEMS. 



INVERTEBRATA. GANGLIATA. 

 MOLLUSCA. 



DIVISION I. MOLLUSCA CEPHALA. 

 Section I. NATANTIA. 

 Class I. CEPHALOPODA. 

 Order I. Nautiliadae. Spirula, Nautilus, and the multi- 



locular Testacea. 



Order II. Sepiadae. (1.) Head surrounded with eight 

 arms and two feet. Sepia, Loligo. (2.) Head 

 surrounded with eight arms, without feet. 

 Octopus, Eledona, Ocythoe. 

 Class II. PTEROPODA. 



I. With a shell. Limacina, Hyalea. 



II. Destitute of a shell. A. Fins double. Pneu- 



modermon, Clio, Cleodora. B. Fin single. 

 Cymbulia. 



Section II. GASTEROPODA. , 



Class I. PULMONIFERA. 



Order I. Terrestrial. 



Subdivision I. Cloak and foot parallel. Arion, Limax, 



Parmacella, Testacella, Veronicellus, Onchidium. 

 Subdivision II. Cloak and foot not parallel. 

 Tribe I. Foot with a lid. Cyclostoma. 

 Tribe II. Foot destitute of a lid. Helix, Bulimus, 



Pupadae, Vitrina, Succinea, Achatina. 

 Order II. Aquatic. 



Subdivision I. Body protected by a shell. 



Tribe I. Shell spirally twisted. A. Shells turreted. 

 Lymneus, Physa, Aplexa. B. Shells depressed. Pla- 

 norbis, Segmentina. 

 Tribe II. Shell conical. Ancylus. 

 Subdivision II. Body destitute of the protection of a 

 shell. Peronia. 



Class II. BRANCHIFERA. 

 Order I. Branchiae external. 



Tribe I. Branchiae exposed. 



A. Branchiae issuing from the cloak dorsally. 



1. Body exposed and destitute of a shell, a. Do- 

 ris, Polycera. b. Tergipes, Tritonia, Mon- 

 tagua, Eolida, Scyllsea, Glaucus, Thethys. 



2. Body concealed in a spiral shell. Valvata. 



B. Branchiae issuing laterally from between the 

 cloak and foot. 



