208 MANUAL OF THE MOLLUSCA. 
Sub-spiral, or scarcely spiral, in melania (Pl. pene Fig. 25*). 
Multispiral, or many-whorled (Fig. 72), as in trochus, where 
they sometimes amount to twenty; the number of turns which 
the operculum makes is not determined by the number of whorls 
in the shell, but by tle curvature of the opening, and the neces- 
sity that the operculum should revolve fast enough to fit it 
constantly. (Moseley.) 
It is said to be articulated when it has a projection, asin nerita 
(Fig. 74). 
Too much importance, however, must not be attached to this 
very variable plate, as an aid to classification ; it is present in 
some species of voluta, oliva, conus, mitra, and cancellaria, but 
absent in others; it is (indifferently) horny or shelly in the 
species of ampullaria and natica ; in paludina it is concentric, in 
paludomus lamellar, in valvata spiral ; in solarium and cerithium, 
it is multispiral or paucispiral. ; 
The researches of Dr. Loyén* haye led to many attempts being 
made to remodel the arrangement of the Gasteropoda by the aid 
of peculiarities in their dentition. Whateverimprovements may 
be thus obtained, it does not appear desirable to introduce a new 
terminology for divisions long since well established, and already 
over-burdened with classical names.t 
The patterns, or types of lingual dentition, are on the whole 
remarkably constant; but their systematic value is not uniform. 
It must be remembered that the teeth are essentially epithelian. 
cells, and like other superficial organs liable to be modified in 
accordance with the wants and habits of the creatures. ‘The 
instruments with which animals obtain their food are of all 
others most subject to these adaptive modifications. and can 
neyer form the basis of a philosophical system.{ 
* Ofversigt af Kong]. Vetensk. Akad. Forhandl. 1847. 
+ The following names were proposed by Troschel (in Wiegman’s Handbuch der 
Zoologie, 1848) and Gray (An. Nat. Hist.) for the principal types of lingna] dentition :— 
a. Teenioglossa, teeth 3.1.3; Littorina, Natica, Triton. 
6. Toxoglossa, teeth 1. 0. 1; Conus, Terebra? 
c. Hamiglossa, teeth 1.1.1; Murex, Buccinum. 
d. Rachiglossa, teeth 0.1.0; Voluta, Mitra? 
e. Gymnoglossa, teeth 0; Pyramidella, Cancellaria, Solarium? 
f. Bhipidoglossa, teeth 00, 1.00; Nerita, Trochus. 
{ The carnivorous opossums have teeth adapted for eating flesh, but are not on that 
account to be classified with the placental carnivora. The lingual teeth, like the 
operculum, usually have a structure characteristic of the genera or sub-genera. Some- 
times they have a general uniform character throughout a whole family or group 
of families. In many cases they present minute differences which promise to be 
valuable aids for distinguishing closely allied species. For example, Patella athletica 
may be distinguished from the common limpet (P, vulgata) by its teeth. 
