214 MOLLUSCA. 



genus Obolella (fig. 166) of Mr Billings is only separated from 

 Obolus by certain internal characters. The little shells belong- 

 ing to this genus sometimes occur in myriads in the Potsdam 

 Group (Upper Cambrian) of North America. 



CHAPTER XX. 

 LAMELLIBRANCHIA TA. 



THE Lamellibranchiata or Bivalve Shell-fish are distinguished 

 by the fact that the head is not distinct, and the mouth is destitute 

 of any apparatus of teeth. The body is more or less completely 

 protected in a bivalve shell, co?nposed of two, usually symmetrical, 

 pieces or valves. There are generally two leaf -like lamellar gills 

 upon each side of the body. 



The Lamellibranchs include all the ordinary Bivalve Shell- 

 fish, such as Oysters, Cockles, Mussels, and the like, and they 

 are all either marine or inhabitants of fresh water. 



Though they agree with the Brachiopoda in possessing a 

 shell which is composed of two pieces or valves, there are, 

 nevertheless, many points in which the shell of a Lamelli- 

 branch is distinguishable from that of a Brachiopod, irrespec- 

 tive of the great difference in the structure of the animal in 

 each. The shell in the Brachiopoda, as we have seen, is rarely 

 or never quite equivalve, and always has its two sides equally 

 developed (equilateral) ; whilst the valves are placed antero- 

 posteriorly as regards the animal, one in front and one behind, 

 so that they are " dorsal " and " ventral." In the Lamellibran- 

 chiata, on the other hand, the two valves are usually of nearly 

 equal size (equivalve), and are more developed on one side 

 than on the other (inequilateral) ; whilst their position as re- 

 gards the animal is always lateral, so that they are properly 

 termed " right " and "left " valves, instead of " ventral " and 

 " dorsal." 



It is to be remembered, however, that many of the Bivalves, 

 such as the Oysters, habitually lie on one side, in which case 

 the valves, though really right and left, are called " upper" and 

 "lower." It is to be borne in mind also that the two valves, 

 especially in the attached Bivalves, may be very unsymmetri- 

 cal, one valve being much larger or deeper than the other. 

 Lastly, there are some cases in which the shell becomes very 



