LAMELLIBRANCHIATA : OSTREID^E. 489 



SUB-SECTION III. 

 THE ORDER OF PTEROPODA, OR PTEROPODS. 



THIS Order also embraces mollusks which inhabit the 

 open sea, where they move in immense swarms, some- 

 Fig. 447- Fig. 449. 

 Fig. 448. 



Limacina antarctica, 

 Hooker. South Polar Seas. 



Hyalea tridentata, Gmel. Clio borealis, Brug. 



Atlantic and Mediterranean. Arctic Seas. 



times leagues in extent. They comprise three families, 

 Hyaleidae, Limacinidae, and Cliidae. 



SECTION III. 



THE CLASS OF ACEPHALA, OR ACEPHALS. 



THIS Class comprises mollusks which have the head 

 obscurely indicated. There are four orders, Lamelli- 

 branchiata, Tunicata, Brachiopoda, and Bryozoa. 



SUB-SECTION I. 



THE ORDER OF LAMELLIBRANCHIATA, OR LAMELLIBRANCHIATES. 



THIS Order contains acephalous mollusks which have 

 their gills in lamellae on the sides, and protected by a shell 

 composed of two valves occupying a similar position, 

 namely, right and left. A single valve with the names 

 of its different parts is shown in Fig. 450, on the next 

 page. There are more than twenty families. 



OsTREiDyE, OR OYSTER FAMILY. This Family com- 

 prises bivalves, of which the common Oyster is a well- 



