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EXPLANATION OF TERMS 



USED IN DESCRIBING THE SHELLS OF TESTACEOUS 

 MOLLUSCA. 



Accessary Valves, Small additional valves placed near 

 the umbones or bosses of the genus Fholas among 

 shells, and on the edges of the pedunculated bar- 

 nacles, among annulose animals. 



Acephalous. Mollusca without an apparent head^ 

 comprehending the bivalves of conchologists. 



Acuminated. Ending in a graduated and often acute 

 point. 



Adductor muscle. That which closes the two pieces of 

 a bivalve together, the base or insertion of which is 

 indicated by an irregular depression in each valve, 

 these being called the muscular impressions. 



Anterior, when applied to the sizes of bivalve shells, 

 is that in which the ligament is not placed. 



Apex, The point or nucleus of a shell ; hence the 

 bosses of bivalves, the points or top of limpets, or 

 of univalve shells, are so called. 



Aperture, or mouth of univalve shells, is the cavity 

 from which the head of the animal is protruded. 



Arcuated, Curved or arched. 



Auricrdated, or eared. A term given to a few bivalves 



which have a flat angulated projection on one or 



both sides of the umbones, or bosses : these processes 



are most developed in the Pectens, and are merely 



c c 4 



