PARTS OF UNIVALVES EXPLAINED. 5 
Nautilus and Ammonites, and which are divided by sept or 
walls. Fig. 3. a a. 
Septe are the partitions which subdivide the internal 
portions of shells of the genera Nautilus, Ammonites, 
Serpula, §c. into distinct chambers. Fig. 3. 5. 
Front, that part of the body in which the aperture is 
situate. Fig. 7. /. 
Volution, one of the turnings or whorls of the spire. 
Fig, 12. e: 
Dextral, or right-handed shells, are those which have the 
aperture situate opposite the right hand of the observer, 
when the specimen looked at is held with the spire perpen- 
dicular, and with the suture, or separating line of the volu- 
tions, ascending from left to right. Fig. 7. 
Sinistral, or left-handed shells, are those whose characters 
are the reverse of species which are dewtral, with the aper- 
ture on the left hand of the observer when viewed in 
front. Fig. 12. 
Operculum, a shelly or horny process attached to the 
foot of the animal inhabiting univalves, and which is in- 
tended to close either wholly or partially the aperture or 
opening of the shell. Fig. 16. 
Varices are longitudinal ribs, which are formed by the 
periodical growth of shells. These have been severally 
the outer lip, to which the animal has added from time 
to time. Varices prevail in the genera Triton, Murea, 
and Buccinum. Some species have never more than a 
single varir. 
Siphunele, a small round tube, which perforates the septa 
in shells of the genera Ammonites, Nautilus, Goniatites, §c. 
Its opening or aperture is situate in the last or external 
volution. Its use is particularly described in the genera 
Ammonites, p. 14, and Nautilus, p. 18. Fig. 3. c. and fig. 
19. ¢. 
Ribs are longitudinal or transverse processes, which rise 
above the surface. Fig. 14. d. 
Teeth are small processes, situated inside the lips in 
several univalve shells, 
Plaits, found principally on the columella of shells of 
Her as genera, as Mitra, Oliva, Voluta, §c. Plate I. 
ge. 2h. e. 
Epidermis, The external skin or cuticle which invests 
many species of univalves, and appears destined to protect 
the shell from injury. 
