EAST COAST MARINE SHELLS 



15 



Fig. 14 



Fig. 15 Fig. 16 



Forms of the operculum 



Fig. 17 



Fig. 18 



MDLTISPIRAL — When there are numerous whorls, 

 often as many as twenty. 



In dextral, or right-handed spiral 

 shells, the opercvilum when spl "al itself is 

 sinistral or left-handed. In left-handed 

 shells the spiral operculum is naturally 

 the reverse. There are a few exceptions to 

 this rule in the Pilidae and Spiratellidae. 



The number of turns in the opercu- 

 lum is not affected by the revolutions of 

 the shell but rather by the form of the 

 aperture with which it must keep pace. 



The operculum is of considerable 

 Interest and always should be preserved 

 with specimens. Not too much Importance, 

 however, should be accorded it as an aid to 

 classification. In Naticidae and Pilidae 

 both horny and shelly operculums occur 

 within the respective families. 



BIVALVES 



The PELECYPODA are entirely aquatic 

 mollusks, although sometimes remaining for 



C K H 



long periods out of water, and rank next to 

 the Gastropoda in variety of forms. The 

 bivalves individually, however, are much 

 more plentiful. 



Normally the Pelecypod shell con- 

 sists of two valves, distinct and separate, 

 usually covering the right and left sides 

 of the animal. 



Margins of Shell . The hinge line, 

 along which the valves are united by the 

 ligament and interlocking teeth, is placed 

 upon the dorsal region of the animal and 

 forms the upper or DORSAL MARGIN of the 

 shell. Fig. 19a. The opposite side is the 

 VENTRAL or LOWER MARGIN, Fig. 19b, and is 

 often thin and sharp. The POSTERIOR or 

 SIPHONAL END, Fig. 19d, refers to the end 

 upon which the ligament is situated, at 

 the opposite extremity is the ANTERIOR 

 MARGIN or ANTERIOR END; Fig. 19e. 



Right and Left Valves . The valves 

 are right or left depending upon the side 

 of the animal. To Identify these the shell 

 should be placed in position as when 

 crawling, the ligament upward and 

 toward the observer, the anterior or 

 opposite end pointing forwards. The 

 right or left valves then agree with 

 the student's right or left hands. On 

 PI. 54, Fig. 8, representing a large 

 clam, the ligament is shown distinct- 

 ly. In this illustration the right- 

 hand point is the anterior end, the 

 lower valve the right valve. 



Fig. 19 

 Parts of bivalve shell, 19a Dorsal margin, 

 19b Ventral margin, 19c Ligament, 19d Pos- 

 terior end, 19e Anterior end, 19f umbo, 19g 

 Position of lunule, 19h Cardinal teeth, 19J 

 Anterior latertil tooth, 19k Posterior later- 

 al tooth, 19/ anterior adductor, 19m Posteri- 

 or adductor, 19n Pallial line, 19o Pallial 

 sinus 



