On the Clausium. 213 



tracted within its shell, this plate nearly covers the aperture at a 

 little distance withia the mouth, and coming in contact with a 

 transverse plait on the outer lip, leaves only a small canal formed 

 between the outer plait and the posterior angle of the mouth, 

 and sometimes an elongated longitudinal plait on the inner lip. 

 When the animal wishes to protrude itself, it pushes the plate 

 on one side into a groove situated between the inner plait and the 

 columella, where it is detained by the pressure of the body of the 

 animal, leaving the aperture free, and when the animal withdraws 

 itself, the plate springs forward by the elasticity of its pedicle, 

 and closes the aperture. This curious structure, and also the 

 plaits of the mouth, which are intimately connected with it, are 

 not formed until the animal has nearly reached maturity. It is 

 best exhibited by breaking off the outer part of the aperture to 

 the distance of about half a whorle, when it will generally be found 

 free; but in order to exhibit it behind the columella in its natural 

 position, when the animal is exserted, it is necessary to kill the 

 animal in that situation, and then suffer it to dry before the outer 

 lip is broken off, and the pedicle will thus become fastened to the 

 side by means of the dried mucus ; it may, however, at any time 

 be relaxed by a little moisture, when it ivill instantly resume its 

 elasticity, and spring from its concealment. 



This part was, I believe, first described* by the accurate Otho 

 Frederic Muller, in his excellent Historia Vermium &c. in 1773, 

 and by him called Ossicula and Scala. He beautifully and accu- 

 rately describes its peculiar function. Draparnaud has since de- 

 scribed it as a new discovery, although well acquainted with Mailer's 

 work, which he quotes frequently ; he does not, however, seem to 

 have been aware of its use. Cuvier also, in his " liegne Animal,^' 

 avows his ignorance of its use. In the Annals of Philosophy for 

 1822, there is a very good description of its mechanism, by Mr. 

 Miller of Bristol, but that gentleman also appears not to havei* 

 seen MuUer's description, as he wishes to take to himself the cre- 

 dit of the discovery both of the Clausium and its use, for having 

 shewn it to Dr. Leach in 1814 previous to the publication of Dia- 

 parnaud's work. 



• It had been before mcntioiud by Daubentoii in liis Dislrihutwn melho- 

 dlqut dei Coquillages, in the Mtmoires de Vyicademit des Seiencen. 



