MOLLUSCA. 145 



already been given. These were the true Gasteropoda of La- 

 marck, while his Mollusca were Hmited to what we have em- 

 braced in the present class. 



The following synopsis may aid in arriving at the genera of the 

 testaceous coverings of such of the Gasteropoda, as are found 

 with us. 



I. Aperture entire, not interrupted in front by a notch or elongated canal. 

 i. Shell shield-like or cup-shaped, aperture very large. 



1. multivalve. Chiton. 



2. univalve. 



* simple. Paiilla, Lottia, An'cylus. 

 ** with a fissure at summit. Cemdria, 



*** with a partial partition, parallel to the aperture. Crepidula. 

 ii. Shell tusk-shaped. Dentalium. 



iii. Outer whorl enveloping all the others, the spire often invisible. Bulla. 

 iv. Shell with an evident spire. 



1. inhabiting the land and breathing air. 



* sub-globular, or wheel-shaped. Helix. 



** ovate, with an elongated spire. Pupa, BuVimus, Succhiea. 



2. inhabiting fresh water. 



* spire wheel-shaped, evident above and beneath. Planorhis. 

 ** spire elevated. 



t aperture ovate. Limnea, Physa, Paludtna, Jlmnicola. 

 tt aperture circular. Valvata. 



3. inhabiting the sea. 



* spire ovate-globose. 



t aperture ovate. 



§ pillar simple. JVatica, Littorlna, Jdnthina. 

 § pillar with an umbilical groove. Lacuna. 

 ft aperture circular. Margarita, Skenea. 

 ** spire elongated. 



§ pillar simple. Turritella, Scalaria, Cingula, Py'ramis. 

 § pillar with teeth or folds. Meldmpus, Tornatella, 

 Odostomia. 

 *** spire ear-shaped. Sigarttus. 

 **** spire irregular, lax. Vermetus. 

 II. Aperture terminating in front by a notch or elongated beak. 

 i. With a notch. 



1. pillar simple. Buccinum, Columbilla. 



2. pillar plaited. Cancelldria. 

 ii. With a canal. 



1. canal short. 



* recurved. Cerithium, Fusus. 



* straight. Rostellaria, Trichotropis, Purpura, Pleurotoma. 



2. canal prolonged. Py'rula, Rmiilla, 



19 



