FOSSIL MOLLUSCA BIVALVES AND UNIVALVES. 243 



One broad fact also strikes the geological student : 

 the specific and numerical abundance of mollusca, 

 bivalve and univalve, is greatest in the most recent 

 formations, and least in the oldest ; whereas the specific 

 and numerical abundance of the Brachiopods is just 

 the reverse greatest in the oldest deposits, and scan- 

 tiest in the latest formed. 



The shells of bivalves may be regarded as hollow 

 cones. The umbo or beak is the apex of such cone. 

 The shells are frequently unequal-sided, one being 

 more elongated than the other. The mouth of the 

 animal within the shell is uniformly on that side of 

 the body indicated by the umbo or beak ; hence this 

 is called the anterior or front part of the shell. The 

 two valves are hinged together by projections and 

 notches which accurately fit each other, and these are 

 technically called " teeth." Their number, size, and 

 position have to be carefully noted, for they are 

 among the chief means of diagnozing genera and 

 species. 



Inside the shells of all bivalves, fossil as well as 

 recent, the student will observe well-defined scars, 

 which indicate where the mantle of the animal was 

 attached. In some mollusca the mantle is in two 

 halves or lobes which are united along their edges, so 

 that a siphon is necessary to admit water to the gills. 

 Such bivalve mollusca are usually sand and mud 

 dwellers, and during life are imbedded in an upright 

 position, with the siphon projecting above the muddy 



