208 MOLLUSCA. LAMELLIBRANCHIA 



others they are always open at some part, and are then 

 said to be gaping ; this gape occurs most frequently at the 

 posterior end, but sometimes also anteriorly. Sometimes 

 the small embryonic shell, known as the prodissoconch, is 

 found at the umbo of the adult shell ; this represents the 

 protegulum of the Brachiopods (p. 170) and the proto- 

 conch of the Gasteropods and Cephalopods. 



In order to be able to distinguish the right and left 

 valves we must determine first the anterior and posterior 

 margins. When the soft parts of the animal are present 

 this is easily done ; but when the shell only is before the 

 observer the points to be noticed are the following : — 



(1) The umbones are generally directed anteriorly ; 



and in inequilateral shells, the posterior part 

 of the valves is, with only a few exceptions 

 (Nucula, Lima), longer than the anterior part. 



(2) The lunule is anterior to the umbones. 



(3) The external ligament is commonly posterior to 



the umbones, and is never entirely in front of 

 them. 



(4) The pallial sinus is posterior. 



(5) When one adductor impression only is present, it 



is the posterior. 



(6) When one adductor impression is distinctly larger 



than the other, the larger is the posterior. 



Having found the anterior and posterior margins, the 

 shell should be placed with the dorsal surface uppermost 

 and the anterior margin pointing away from the observer, 

 then the right and left valves will be on his right- and left- 

 hand sides respectively. 



