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. 44 8. 



Limacina antarctica, 

 Hooker. South Polar Seas. 



Hyalca tridentata, Gmel. 

 Atlantic and Mediterranean. 



Clio borealis, Brug. 

 Arctic Seas. 



times leagues in extent. They comprise three families,- 

 Hyaleicke, Limacinidse, 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 lamellas 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. 



OSTREID/E, OR OYSTER FAMILY.- -This Family com- 

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



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