160 MOLLUSC A. 



Of the fossil species, many are congeneric with existing forms, 

 and some of them are of comparatively large size, e.g. Conularia, 

 which is sometimes nearly two feet in length. The Silurian Ten- 

 faculties, " though often referred to the Tubicolar Annelides, ap- 

 pear to belong, without doubt, to the Pteropoda." 



Hyalceidce. Hyalaea= Carolina. Thecidcs. 



Triptera. *Conularia. 



Cleodora. Cymbuhtda. 



Balantium. Cymbulia. Limacinida. 



Styliola Creseis. Halopsyche= Psyche. T . 



Diacria. Tiedemannia. 



Spiriahs. 



*Tentaculites. 



Class IV. GASTROPODA. 



BRANCHIOGASTEROPODA. COCHLIDES. PLATYPODA. 



Land or water mollusks, generally encased in a univalve shell. 

 Locomotion effected by the ventral disk or foot. A distinct head 

 in nearly all, with one or two pairs of tentacles. Dioecious or 

 hermaphrodite. 



A heart, liver, and convoluted intestine are mostly present : 

 the mouth is provided with a radula. The eyes are never more 

 than two, either placed on the tentacles, or more frequently sessile 

 at their base ; but they are absent in Scaphopoda and CbitonidEe. 

 There is a distinct organ of hearing, consisting of two round 

 vesicles containing ciliated otolites, remarkable for their oscil- 

 latory action in living or recently killed animals. 



The shell is rarely hidden in the mantle, and except in Chito- 

 nidae it is invariably single, and most frequently spiral. 



Heteropoda are sometimes included in this class. Scaphopoda 

 are placed by Hackel and Huxley with Pteropoda ; but Glaus 

 ranks them as one of the four classes of Mollusca. 



There are said to be upwards of 22,000 species in this class. 



Head rudimentary SCAPHOPODA. 



Head distinct. 



Respiration by branchiae. 



Branchiae arborescent OPISTHOBRANCHI ATA. 



Branchiae pectinate PROSOBRANCHIATA. 



Respiration by a pulmonary sac PULMONIFEEA. 



