LYMNAID/ OF NORTH AMERICA. ) 
American Lymnzas. The median projection may be wide (as in ob- 
russa (F) and caperata (L)) or very narrow and acute (as in umilis 
(H) and catascopium (K)). The jaw also varies somewhat with age 
and wear (compare catascopium (KK) with a worn jaw of the same 
species (M) ). The lateral margins are also more produced in some 
species (K) than in others (B). 
Cc. THE RADULA. Plate III. 
The radula in Lymneza 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 radule of Lymnza. 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.” 
The following remarks of Dr. Pilsbry are also of interest in con- 
nection with the study of the Lymnzid 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 radulez 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 Lymnzas are always multicuspid or 
serrated, showing a descent from the Tectibranchiate stock of marine 
mollusks. The Lymnezid type of radula is thus a good example of 
the law of mesometamorphosis, as explained above. 
In the Lymnzeas 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 (pl. III, 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 radule of the Lymnzas fall into two main types: First, 
those species with bicuspid laterals (as stagnalis, reflexa), and, second, 
those species with tricuspid laterals (as columella, auricularia). The 
1Guide to Study of Helices, p. xiii. 
