50 UNIVALVES. 



In the ordinary radula the ribbon consists of five rows of teeth, 

 the central or rachidian, the laterals on either side, and the marginals 

 at each edge. The rachidian is generally present ; some genera 

 have lost both it and the laterals ; some have lost both laterals and 

 marginals. One genus has but one tooth ; in others the number 

 runs into thousands. Buccinum undatum, for instance, has about 

 250, Limncea stagnalis has over 8,000, Helix aspersa has about 15,000 ; 

 in some of the Mediterranean species there are nearly three-quarters 

 of a million. The radula is now used largely as a basis of classifi- 

 cation. The Toxoglossa, represented in the British list by Mangilia 

 and Pleurotoma, the ribbon is made up entirely of the marginals, 

 which are largely developed. The Rachiglossa, represented in our 

 list by Trophon, Murex, Purpura, Lachesis, Columbella, Nassa, 

 Buccinum, Buccinopsis, Fusus, and Marginella, there is a central tooth 

 with a single lateral. The Taenioglossa, comprising the species we 

 have numbered from 370 to 460, that is, from Natica to Triton, have 

 a rachidian, a lateral, and two marginals. In the Ptenoglossa, 

 represented in our list by lanthina, Scalaria, and Aclis, the radula 

 consists of an indefinite number of hooked teeth with the largest on 

 the margin. In the Rhipidoglossa, of which the British genera are 

 Fissurella, Puncturella, Emarginula, Hahotis, Scissurella, Cyclostrema, 

 Trochus, Phasianella, and Neritina, the laterals vary in number from 

 nine to three, and the marginals are many and large. In the 

 Docoglossa, represented with us by Tectura, Lepeta, Propilidium, 

 Patella, and Helcion, the laterals and marginals are occasionally 

 wanting, and the teeth are thick and beam-like. In Lepeta and 

 Propilidium there is the usual central tooth, but in Patella this is 

 replaced by four teeth, and in other genera by two. The ribbon in 

 this group is lengthy ; that of the common limpet is made up of 

 180 rows. This, however, is not the extreme length known; the 

 periwinkle, for example, belonging to the Tsenioglossa, has a radula 

 2^ inches long, containing 600 rows. For the purpose of describing 

 these molluscan teeth concisely, a dental formula has been adopted 

 on the ordinary lines ; Lepeta, for instance, is given as 2, o, i, o, 2, 

 meaning 2 marginals on each side, no laterals, and i central tooth. 

 In the other orders the teeth also differ in number and character, 

 but this rapid glance at the seven sub-orders of the Prosobranchiates 

 must suffice us. 



The Prosobranchiates derive their name from the veins con- 

 nected with the branchiae being situated in front of the ventricle of 

 the heart, those of the Opisthobranchiates being placed behind the 

 ventricle. All the mollusca have a heart ; sometimes it consists of 

 a single auricle and ventricle, as in the Monotocardia ; sometimes 

 there are two auricles, as in the Chitons and the Diotocardia; some- 

 times there are four, as in the Nautilus ; in all cases the number of 

 auricles being the same as that of the branchiae, by which the blood 

 is aerated as it passes on its way. 



The land mollusca breathe air, the water mollusca breathe 

 water ; but the freshwater mollusca that are without an operculum 

 breathe air, though they live in the water, and take it down with 

 them in bubbles on their visits to the surface, while the water 

 mollusca that live within the tide range retain enough water for 

 them to breathe until the return of the tide. Thus the mollusca 



