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 sinzstral, sometimes also heterestro- 
phe or heteroclite, and are generally called by dealers unique. ° 
In the devtral 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 trouvé animal au moment de sa _nais- 
sance, et d’un obstacle qu'il a trouvé lorsqu’il a voulu tour- 
ner sa téte du cété que la nature lui a indiqué.” This ex- 
planation might have been received, had such changes in 
