ON MOLLUSCA. 74 
we have been just speaking. But even in those in which the 
intermissions of the growth are not so sensible, and are marked 
only by simple striz, the termination of the growth is very 
often indicated by a swelling more or less thick, sometimes 
simple, sometimes denticulate, and which is equally formed of 
a vitreous substance. It is also at this epoch, that in the 
univalves the vitreous substance of the interior depot, and even 
exterior, as in the porcelaines, increases, thickens, appears as 
it were to extravasate, and tends to diminish the aperture, the 
form of which it very often changes, as may be seen in the true 
cassides and certain helices, so as sometimes to join the two 
edges, and form a sort of continuous peristoma. ‘The orifice 
of a univalve shell is also often modified by the formation of 
teeth, not only at the internal side of the right edge, but again 
upon the left edge, and the columella itself. These teeth are 
evidently produced by the cannelures of the mantle which 
accompany the pedicle, that joins the foot of the animal to 
the tortuous part of its body. 
The explication of the formation of sinuses, notches, emar- 
ginations, is similar to that of tubercles, canals, swellings, and 
varices, with this difference, that these solutions of continuity 
in the edge of univalves or bivalves, are owing to some move- 
able projecting part, which is alternately put forth and with- 
drawn. ‘Thus, in the univalve shell, the anterior emargination 
of the aperture is, as we have already had occasion to observe, 
owing to the tube formed by the edge of the mantle. ‘The 
sinus which is sometimes remarked in the anterior part of the 
right edge, as in pterocera and strombus, results from the pas- 
sage of the head; the medial or sub-posterior notch of the 
same edge, and which is found in pleurotoma, and in many 
other genera, is referrible to the exsertion of the female organ 
of generation or the oviduct. In all probability this does not 
exist except in female subjects. It is at least certain that this 
disposition has not been hitherto found except in the dicecious 
