254 
ALIMENTARY SYSTEM — MONOGNATHA. 
erroneously, evidently regards the palatal plate of the Elasniognatha 
as not honiologons with the mandibular palatal extension found in 
other monognathous groups. 
In the monognathou.s species the mandihle is opposed by the divided 
lower lip, which acts the part of a lower mandible in grasping the 
substance to be devoured and enabling the odontoiihore to rasp off 
particles of food, which are carried to the oesophagus by its action. 
d'he Monognatha may be ranged under five chief groups, viz. : — 
Pycnognatha, Odontognatha, Leioguatha, Oxygnatha, and Blasnio- 
gnatha, which are re.spectively characterized by the absence or presence 
of several more or less vertical thickenings or ribs, more especially 
upon the anterior surface of the jaw, by the development of a median 
projecting beak or by the possession of a large (piadrangular accessory 
palatal plate. 
'I’he Pycnognatha (tti'ki'os', crowded ; yi'd^^o?, jaw) often possess 
a somewhat convexly arcuate jaw, rvithout the median beak or projec- 
tion. It is e.specially characterized by the numerous tine and delicate 
vertical rilis, sometimes covering the 
whole anterior surface of the jaw, 
though occasionally only i)erceptil)le 
towards the cutting edge, which thus 
, /• 1 14 Ku'.. ’>08. -A Striated or Pycno.^natlious 
necoines niiely crenulato. Anion^'st mandible, x j/c/lv / u/c/u'/Za urn., 
‘ from vSeamer, near Scarloroii?li, collected 
our inono^natlious fetyloinniatojmora, UyNIr. J.A. Hargreaves. (Photographed 
‘ , by Mr. 'I'. W. Thornton from a prepara- 
this type of mandihle is probably the 'w Mr- J- w. Neville.) 
tirst .stage in the .siiocialization of the julmitive comim.site jaivs ivhich 
still characterizes our and other siiecies, this moditica- 
tion arising from the fusion of the originally sepiirated plates, the 
over-la])iiing lines of junction giving rise to the characteristic vertical 
stria', although, as suggested by Pilshry, this condition may also he 
reached through the odontoguathous stage by the degeneration of the 
ribs. 'I’he Ihdim'nti and some Helices are e.xamples of this group. 
'I'he Odontognatha (oSoih-, tooth ; yrd^o?, jaw) is rvell illus- 
trated by the Arionhlir and many IJ('Hcl(hv and embrace those 
species whose jaws hear, esj)ecially ujion their anterior surface, 
more or less numerous and prominent ])arallel or slightly convergent 
ribs, which ])roject beyond and denticulate the free margin, and 
sometimes the upi)er margin also. 'I’hese ribs and their denticula- 
tions are usually more distinctly develo])ed where their number is 
limited and less .strongly marked when more numerously pre.sent, 
