EXPLANATION OF TERMS. 



Acephalous, Mollusca without an apparent head ; com- 

 prehending 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. (See 

 Muscular Impressions.) 



Annulated. Marked with rings. 



Anterior. The side of the shell, in Bivalves, where the 

 ligament is situated. (See Posterior.) 



Aperture. The mouth or opening of the shell, in Uni- 

 valves, from which the head of the animal protrudes. 



Apex. The commencement or top of the spire in Uni- 

 valves. 



Articulated. Jointed. 



Auriculated. {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. 



Auriform. (Ear-shaped.) Shaped like an ear ; as the 

 Haliotis. 



u 3 



