208 MANUAL OF THE MOLLUSCA. 



Siih-spiral, or scarcely spiral, in melania (PI. VITI., Fig. 25*). 



MuUispiral, or many-whorled (Fig. 72), as in trocJius, 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 the 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) as in 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 valuta, 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. Loven* have led to many attempts being 

 made to remodel the arrangement of the Gasteropoda by the aid 

 of peculiarities in their dentition. Whatever improvements 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. f 



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 

 never form the basis of a philosophical system. ;{: 



* Ofverslfft af Kongl. Vetensk. Akad. Furhandl. 1847. 



t The following names were proposed by Troschel (in Wiegman's Handbvch der 

 Zoologie, 1848) and Gray (An. Nat. Hist.) for the principal types of lingual dentition: — 



a. Tsenioglossa, teeth 3. 1.3; Littoriua, Natica, Triton. 



b. Toxoglossa, teeth 1. 0. 1; Conus, Terebra? 



c. Hamiglossa, teeth 1. 1. 1 ; Murex, Buccinum. 



d. Kachiglossa, teeth 0. 1. 0; Voluta, Mitra? 



e. Gymnoglossa, teeth ; Pyramidella, Cancellaria, Solarium? 

 /. Ehipidoglossa, teeth 00, 1.00; Nerita, Trochus. 



X The carnivorous opossums have teeth adapted for eating flesh, but are not on that 

 account to be classified with the placental camivora. The lingual teeth, like tlie 

 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. 



