SErT. 1, 1868.] 



HARDWICKE'S SCIE NCE-GO SS1P. 



201 



variously curved, angular, or rarely straight. The 

 number of teeth in the transverse row is nearly con- 

 stant for the same species, and the number of rows 

 is exceedingly variable in different species : longi- 

 tudinally the teeth are usually arranged in a 

 triple series, and each transverse row is but a repe- 

 tition of the rest. The central area is called the 



Fig. 2f)7. Teeth of Paludina vivipara. 



rachis, and the teeth form usually a single series ; 

 the lateral areas are called the pleura, the teeth 

 on which are termed uncini, and. 

 usually are extremely numerous. 

 The term laterals is employed in 

 a restricted sense to designate a 

 series of teeth intermediate be- 

 tween the rachidian and the uncini. 



The lingual ribbon of Paludina 

 vivipara consists of a few trans- 

 verse rows, each composed of a 

 central oval tooth (a), slightly hooked and denti- 

 culated ; and three nearly similar uncini (b, c, d), 

 which are oblong and toothed on the upper sides : 

 the number of teeth in each transverse row is there- 

 fore seven. 



The number and arraugement of the teeth are 

 capable of easy representation by a numerical 

 formula. Thus, 3. 1. 3 represents the system in 

 Paludina, signifying that each transverse row con- 

 sists of one median or rachidian tooth, hanked on 

 each side by three uncini. 



In Vitrina the general formula is oo.-l.oo; where 

 00 represents 37 ; and as there are 100 rows, the 

 lingual teeth of Vitrina are, 7,500 ; the formula 

 will now stand ^~ = 7,500. In the great slug, 

 Limax maximus, there are 28,800 teeth, dis- 

 tributed in 160 rows of 180 teeth in each. The 

 number of teeth has no relation to the size of the 

 animal ; thus, Helicella cellaria possesses 1,330, 

 while U. nitidula, less in size, has nearly three times 

 that number. 



The teeth of the fresh-water Gastropods are 

 characterized by their fewness, whilst those of 

 air-breathing snails are remarkable for their ex- 

 traordinary number. 



The lingual ribbon of Paludinidte is very simple, 

 and presents the same general features throughout 

 the family, and places it in close proximity to the 

 Periwinkles and others comprised in the family 

 Littorinidce. 



The general formula is 3. 1. 3 ; the number of 

 transverse rows is very limited. 



The lingual ribbon of Valvata piscinalis is long ; 

 the central tooth {a, fig. 20S) is subquadrate, with a 



Fig. 208. Teeth of Valvata piscinalis. 



produced base, hooked and denticulated ; the three 

 uncini (b, c, b) are lanceolate, and toothed on each 

 side. 



The lingual teeth in NeritinafluviutiUs (fig. 209) : 

 — The central tooth (a) is minute ; the first lateral 

 tooth is large, subtriangular, succeeded by two 



Fig. 209. Teeth of Neritina fluviatilis. 



very minute ones; the uncini are about sixty in 

 number ; the first one is very large, and of a re- 

 markable shape ; the rest are very slender, hooked, 

 and denticulated. 



The teeth of Assiminia Grayana are seven in 

 number ; the central with a base produced into a 

 horn, with five to seven pointed lobes, the first 

 lateral tooth with seven lobes. The second is slender, 

 claw-like, and serrated ; the third is rounded at the 

 tip, with minute denticulations (fig. 210). 



Fig. 210. Teeth of Assiminia Grayana. 



Testacella, and the other predaceous Pulmonifera?, 

 do not possess horny jaws. The lingual ribbon is 

 very large and wide, composed of about fifty trans- 

 verse rows, which are oblique, and descend towards 

 the middle ; the teeth are conical, regularly curved, 

 barbed at the point, and having a projection 

 on the middle of the posterior side, from which 

 the remainder thickens ; the teeth diminish in 

 size towards the centre ; there are 51 in eacli 

 row. 



