ALIMENTARY SYSTEM — MONOGNATHA. 
253 
irregular contour in immature animals, in the adults presents a 
distinct median notch, with usually a broad flat projection at each 
side and a more prominent angle towards the outer margins, the 
projecting angles indicating the marginal terminations of four slight 
and indefinite rib-like thickenings which can in some lights be 
occasionally detected. The acute ends of the dorsal jaw are firmly 
attached to the pointed extremities of the almost linearly crescentic 
lateral jaws, which are placed at right angles to the upper jaw and 
have their concavity outwardly directed. 
In action the dorsal and lateral jaws do not move simultaneous!}', 
but the upper jaw is first brought down and the side jaws close up 
laterally. 
The i\IoNOGNATHA (/xduos, single ; yrctdos, jaw), or mollusks with a 
single mandible, usually possess a more or less semilunar or crescentic 
jaw of a hard and chitinous nature, placed dorsally at the entrance to 
the oral cavity and varying in consistency from a very thick to quite 
a delicate substance, and from a deep opaque brown to a translucent 
amber, the colour being always deepest along the free cutting edge ; 
but in some groups, as Pupa, the entire jaw has the appearance and 
colour of cartilage. 
Fig. 506. — Oxygnathous jaw of L'nnax 
inaximiis L., in process of development, 
showing its bilateral origin, magnified 
(after Wiegmann). 
Fig. 507. -Mandible of Helix ptdchclla 
Miill., X 40, Seamer, near Scarborough, 
collected by Mr. J. A. Hargreaves, 
showing the posterior palatal extension. 
(Photographed by Mr. T. \V. Thornton 
from a preparation by Mr. J. W. Neville.) 
The Monognathous jaw really originates, according to Wiegmann, 
as two separate lateral parts, which afterwards unite in the median 
line, thus furnishing further corroborative evidence of the derivation 
of the monognathous jaw from a more ancient ty])e, composed of a 
greater number of parts. 
From the posterior lower margin of the jaw there arises a partially 
chitinous plate or membrane, which extends taperingly backwards 
upon the roof of the buccal cavity, gradually blending with it, and 
doubtless strengthening the jaw and its cutting edge ; this feature 
attains its maximum development in the distinctly defined palatal 
plate of the Elasmognatha. Mr. W. G, Binney however, as I think 
