ALIMENTARY SYSTEM TETRAGNATIIA. 
251 
The polyplacognathoiis or composite jaw is an excessively ancient 
and primitive type, jirobably especially characteristic of a once almost 
nniversally dispersed and more uniform molluscan fauna than any 
existing at the i)recent day. 
This type of jaw is, however, still retained by a limited number of 
holarctic species and by Laomu, a helicoid genus inhabiting the 
Southern Hemisphere, these few species at the present time probably 
solely constituting the now widely scattered and isolated remnants 
of this ancient fauna, in whose surviving members the specialization 
of the buccal armature has become arrested and has therefore lagged 
more or less behind that of the other organs of the body. 
The Pleurognatiious species embrace the (iuadrimaxillate and 
biniaxillate forms and practically includes all our Streptoneures ; the 
paired lateral chitinous jaws, though usually distinctly separated, are 
in Ldinellarki and other marine species manifestly fused together 
dorsally, forming a single piece. They are thickest at the free inner 
edge, more gradually blending with the muscles at the outer margin, 
with smooth or characteristically sculptured surfaces, the peculiarities 
of which may differ greatly in the vai’ious species. They are placed 
somewhat horizontally at each side of the buccal cavity, but working 
against each other; they are probably, however, functionally feeble 
and serve more especially as points of attachment to the various 
muscles actuating the radula and buccal bulb generall3L 
Tbe Tetr.ygnatiia (rerpa-, four ; yvaSo^, the jaw) or (^[uadrimaxillate 
species are exem})liffed by Cydostoina 
elegans, in which the paired lateral 
jaw's are placed on each side of the 
radula, each lateral pair being com- 
l)Osed of a small and somewhat cunei- 
form posterior segment more or less 
intimately adherent by its contiguous 
margin to the anterior and larger 
one, which is of an irregularly oval 
form, with sinuate inner margin and 
a somewhat hexagonally papillate 
surface, the papillre being arranged 
in soniewbat regular series, maiidy 
tending to converge towards the sinus on the inner margin, to which 
side this peculiar sculpture is more particularly restricted. 
Fig. 503. — Pleurognathous Qiiadri- 
maxillate mandibles of Cyclostoma 
elegans (Miill.), from Box Hill, Surrey, 
collected by l\Ir. W. Whitwell, X 12, 
showing the peculiar sinuation of the 
inner margins and the irregularly hexa- 
gonal sculpture of the larger segments 
(photographed by Mr. T. W. Thornton 
from a careful preparation by Mr. J. W. 
Neville). 
