ALIMENTARY SYSTEM — MONOGNATHA. 
255 
but young shells have fewer and less prominent ribs and denticles 
than older specimens, the median ribs first appearing and development 
following on towards the extremities of the jaw, thus being analogous 
to the growth of the teeth in man, 
whose incisors or cutting teeth first 
appear, these being followed by the 
molars, etc., and lastly by the wisdom 
teeth. In addition to these well- 
marked ribs, there are numerous fine 
vertical strim and also incremental 
strim or lines of growth, which are parallel with the cutting edge. 
Mr. Crowther has shown that the strength of the jaw and the 
number of the ribs may be modified by the environment, a soft, 
succulent herbage developing few ribs on a comparatively delicate jaw, 
while the same species living on coarser fare by dusty road-sides have a 
harder jaw with stronger and more numerous denticles, but the 
immature animals always possess a more delicate jaw, with fewer and 
slighter ribs, than their mature companions in the same locality. 
The Leiognatha (Acto?, smooth; yvido^, jaw) typically possess 
a smooth mandible or jaw, from which 
all the striation has become obliterated 
and Avhich show no trace of the 
median rostrum or beak, which is so 
striking a feature in the oxygnath jaw 
and may be viewed as the antithesis 
of the odontognatha ; in Leiognatha 
the jaw has lost all trace of ribs or stria} 
and is ai)parently of homogeneous composition, while in the odonto- 
gnatha the ribs and denticulations have assumed great prominence. 
The Oxygnatha (o^ih, sharp ; yvaOos, jaw) are exemplified by 
the Lhnacidw and Vitriuidce, which possess a jaw strongly arcuate 
from front to back, but smooth on 
the surface, owing probably to the 
disappearance of the delicate rihlets, 
characteristic of the pycnognatha ; 
they are, however, more especially fig. 511 .- a Rostrate or o.\ygnathous 
. . -Ill 1 • • 11 mandible, Limax flavus L., X 10, 
distinguished by being vertically Christchurch, Hants. 
carinate in the centre, the keel terminating in a median beak or 
projection on the cutting margin. 
Fig. 510. — An Unstriated or Leio- 
gnathous mandible, Pupa cylindracea 
(Da Costa), x 40, Bev’erley, collected and 
prepared by Mr. J. D. Butterell. 
Fig. 509.-A Ribbed or Odontognathous 
mandible, Helix poinatm L., x 10, from 
Preston Candover, Hants., collected by 
the Rev. H. P. Fitzgerald. 
