138 



SYSTEMATIC ARRANGEMENT 



teeth, with many (about sixty) series of very slender, declivated 

 hooks ; the central tooth minute, subquadrate ; the first lateral large, 



Fig. 73. — Teeth of Neritella fluviatilis. 



subtrigonal, scapula-shaped, with the front edge rather short, repli- 

 cate ; the second and third minute, between the first and fourth ; 

 the fourth very large, coloured, irregular hood-shaped, with an odd- 

 shaped base transversely produced externally into a fulcrum. — Rossm. 

 Icon. iii. 34, fig. 



Central tooth of Neritella fluviatilis minute, unarmed, body sub- 

 quadrate, narrowed at the base, produced; lateral teeth three; of which 



Fig. 74. — Teeth of Nerifella crepidularia. 



the first is very large, laminaceous, transverse, snbtriangular ; upper 

 margin rather straight, folded back, flat, the lower outwardly rounded, 

 interiorly notched ; second and third minute, suboval, laminaceous ; 

 middle keel thicker, flexuous : lateral series of teeth about sixty ; 

 the first by far the larger, very irregular in shape, scape short, thick, 

 furnished on the inside with a tubercle, head very large, transverse, 

 capped, convex, anterior margin radiato-denticulated, along with the 

 scape emitting a large wing which is rounded and recurved ; other 

 lateral teeth equal, with a suboval hook denticulated behind ; scape 

 produced on the inside (fig. 74). 



Dr. Shuttleworth believes that the erosion of the apex of some of 

 the fluviatile Neritella is caused by the animal's filing off the apices 

 by its teeth, the mark of the teeth being left on the surface of the 

 erosion. This is particularly the case with the specimens which 

 live in streams in granitic countries, and he suspects that the animals 

 supply themselves with the calcareous matter to form their shells 

 from this source ; the body of the Neritellce being withdrawn from 

 the tip of the shell as the shell is enlarged in size. 



Neritella picta is strictly marine : on sticks and stones in a grove 

 of trees a little above half-tide level ; also in dirty places on rocks at 

 a little below half-tide level (C. B. Adams) ; but according to Mr. 

 Cuming, on a mud-bank partially overflowed with fresh water ! 



