LYMN^EIDiE OF NORTH AMERICA. 9 



American Lymnaeas. The median projection may be wide (as in ob- 

 russa (F) and caperata (L)) or very narrow and acute (as in humilis 

 (H) and catascopium (K)). The jaw also varies somewhat with age 

 and wear (compare catascopium (K) with a worn jaw of the same 

 species (M) ). The lateral margins are also more produced in some 

 species (K) than in others (B). 



c. the radula. Plate III. 



The radula in Lymnaea is strap-like or ribbon-like, as in the other 

 groups of the Pulmonata, the basis of attachment being subquadrate 

 or quadrate in shape. It is important to remember the law of meso- 

 metamorphosis in the study of the radulae of Lymnaea. This law, as 

 recorded by Dr. Pilsbry, is as follows: ''All modifications in the teeth 

 proceed from the median line of the radula outwards toward the edges, 

 the outer marginal teeth being the last to be modified." 1 



The following remarks of Dr. Pilsbry are also of interest in con- 

 nection with the study of the Lymnaeid radula : "A study of the mar- 

 ginal teeth, therefore, gives a clue in many cases to the ancestral con- 

 dition of a much modified radula ; although in certain groups the change 

 has been so long established and has proceeded so far that even the 

 outermost teeth no longer retain their primitive form. In such cases 

 recourse must be had to the radulae of young individuals or embryos 

 still unhatched, which sometimes retain an ancestral type of teeth. (See 

 also Sterki, 1893, plate X.) 



"The evident reason why the order of tooth-changes stated above 

 should obtain is that the median portion of the radula is the part most 

 used on account of its position and the convex boss-like shape of the 

 subradular cushion." 



The marginal teeth of the Lymnaeas are always multicuspid or 

 serrated, showing a descent from the Tectibranchiate stock of marine 

 mollusks. The Lymnaeid type of radula is thus a good example of 

 the law of mesometamorphosis, as explained above. 



In the Lymnaeas the teeth are divisible into two distinct series, 

 the laterals and the marginals. Between these two series there is a 

 third, which combines the characteristics of both laterals and marginals, 

 which are called intermediate teeth (pi. Ill, fig. A). There are gen- 

 erally from eighty to over a hundred rows of these teeth, which overlap 

 in the usual pulmonate manner (fig. B). 



The radulae of the Lymnaeas fall into two main types: First, 

 those species with bicuspid laterals (as stagnalis, reflexa), and, second, 

 those species with tricuspid laterals (as columella, anricnlaria) . The 



Kiuide to Study of Helices, p. xiii. 



