62 MOLLUSCA 



similar organization. We consider this division of the uni- 

 valves into canaliculated and entire, as obvious and natural. 



The next character, in point of importance, appears to 

 depend on the direction of the revolutions of the spire. In 

 general, when a spiral univalve is placed upon its base, or 

 mouth, with its summit towards the observer, the mouth 

 will open on the right side of its axis or pillar, and the 

 whorls will be observed to revolve from right to left, be- 

 ginning at the base, and ending at the summit. These 

 shells are termed dextral. In a few shells, however, this 

 order is reversed. The mouth^occurs on the left side of the 

 pillar in the above-mentioned position, and the whorls from 

 the mouth to the summit revolve from left to right. Shells 

 of this sort are termed sinistral, sometimes also heterostro- 

 phe or heteroclite, and are generally called by dealers unique. 



In the dextral shells, the animals have the external open- 

 ings of the rectum, penis, and uterus, on the right side of 

 the body, and the heart on the left. In the sinistral shells, 

 these organs are placed on the opposite sides. Thus the 

 openings of the rectum and organs of generation are on the 

 left side, while the heart is situated on the right. Here 

 again we have an external character impressed on the shell, 

 which indicates certain arrangements in the organs of the 

 animals. We are aware that some conchologists consider 

 the sinistral shells as accidental varieties, and on that ac- 

 count regard the character which is indicated as of inferior 

 importance. Bosc, indeed, says, " La cause de cette varia- 

 tion dans la direction des spires, vient des circonstances 

 dans lequelles s'est trouve 1'animal au moment de sa nais- 

 sance, et d'un obstacle qu'il a trouve lorsqu'il a voulu tour- 

 ner sa tete du cote que la nature lui a indique." This ex- 

 planation might have been received, had such changes in 



