ALIMENTARY SYSTEM — DIGNATIIA AND TRIGNATIIA. 
Fig. 50L — Pleurognatlious Inmaxillate 
maiulil)les of Bythinia tcntaculata 
X from canal, near Noltingham, 
collected by Mr. A. CL Stubbs, showing 
the peculiar inytiliform shape and smooth 
unsculplured surface (photographed by 
Mr. T. \V. 'J'hornton from a careful pre- 
paration by Mr. J. W. Neville). 
Tlie Dignatii.v two : ■ ynWo ^, jaw) are by far tlie most numerous 
iStreptoneurous group, embracing- all the bimaxillate species, of which 
IhitJiin'm tcntdcnldtn may be taken as typical. Jjike Cijclostonut , 
the Ihithinlw are often stateil to be 
entirely destitute of jaws, but they 
really })ossess two, one at each side 
of the buccal cavity, both of which 
are of a somewhat inytiliform sliaiie, 
almost jierfectly transparent and 
totally without noticealde surface 
sculpture. The smaller segments of 
the (piadrinia-xillate jaws liave in the 
Dignatba become lost or perliajis fused 
with the larger anterior segments, 
which are now alone jiresent. 
Certain extra-British tmlmonate siiecies are, according to Lang, 
dignathous, althongh not pleurognatlious, the jaws being })laced on 
the roof and Hoorof the entrance to the buccal cavity. 
In Vdlntfd, however, which in many features is of very luhnitive 
organization, there is a vestigial indication of an upper jaw, and it 
thus links together the tri-niaxillate hermaphrodite forms with the 
hi-maxillate dimeious ones, forming another curious feature in its 
.'^omewliat anomalous organization. 
The TuKiN.vrii.v [rpl-, three; yr«dos', jaw) or trimaxillate species 
are, in this country, practically confined to the Limna hhc, and are 
characterizeil h}^ the iiossession of a 
well-develoiied transverse upper jaw, 
with sometimes a ju-omiiience upon the 
free edge and a pair of lateral jaws which 
are generally much smaller and slighter, 
and sometimes alnio.st vestigial, hut in 
some of the species of P/anorhis and 
A/ici/his the dorsal and lateral limbs 
have ajiparently become more or less fused together and thus form a 
single horse-shoe sha]»ed buccal plate. 
Ill Linuum pryet/nt, which may he regarded as a typical representa- 
tive of this groiif), the well-developed, convex and somewhat 
lenticular upper or dorsal jaw is of a reddish horn-colour, hut 
blackish along the free cutting edge, which, although exhibiting an 
4 
i 
Fig. 505. — 'I'rignalhous clorso- 
j)leural mandibles from Linnnea 
peregra (Mvill.), X 20, Cliristchurcb, 
Hants. 
