PKOBOSCIDIFERA. 69 



orders of Pectinibrancliiata, Scutibranehiata, Cyclobran- 

 chiata, and Tubulibranchiata, the two latter appearing, 

 however, to be modifications of the second. 



Order PECTINIBRANCHIATA. 



Gills comb-like, formed of one, or rarely of two, longitu- 

 dinal series of laminae on the left side of the mantle over 

 the back of the neck. Animal unisexual. Shell spiral. 



The Pectinibranchiate Gasteropods offer to our contem- 

 plation a vast assemblage of Mollusks remarkable for the 

 extreme variety in the form of their shelly envelopes, but 

 well associated by Cuvier on account of the comb-like 

 nature of their gills. They have been divided by Lamarck 

 into Zoophagous and Phytophagous, the former being sup- 

 posed always to possess a siphon and emargination in the 

 mouth of the shell, and the latter being without either. 

 This arrangement, however, does not hold good, and they 

 are here divided into two sections from the form of the 

 mouth. 



Sub-order PKOBOSCIDIFERA. 



Head small, with an elongated, retractile, longly exsertile 

 proboscis, when retracted hidden within the body ; tenta- 

 cles close together at the base, or united by a veil over the 

 base of the proboscis ; eyes sessile, on the outer base of the 

 tentacles. Operculum annular (except in Natica). Carni- 

 vorous, eating living mollusca and other animals. 



The predaceous tribes of Prosobranchiate Gasteropods 

 are said to form the round holes so commonly found in other 

 shells ; the lingual membrane is placed near the apex of the 

 exserted trunk ; the trunk or proboscis is of a very compli- 



