ALIMEN T All Y S Y STEM — POLY PL ACOGN AT HA. 
2j0 
Out Gastropoda maybe eonveuieutly grouped for study as Agiiatlui 
and Giiathopliora, according as they do or do not possess this buccal 
armature. 
The Agnatiia (<«, Mithout; yi’ddo'j, jaw) embrace those species 
entirely destitute of definite or distinct jaws, and are limited in 
this country to the species of the genus TedacelUi, although, even 
in that group, the upper or dorsal mandible is represented by an 
indistinct chitinous thickening. 
The Gnathophor.i (ycddo?, jaw; to bear), or jaw bearei's, 
exhibit great diversity in the character and disposition of their 
mandibular ai)paratus and, in accordance with the number of their 
constituent parts, may be conveniently divided into five principal 
groups or sections, distinguished as Polyi)lacognatha, Tetragnatha, 
Dignatha, Trignatha, and iMonognatha, although the peculiarities 
characterizing these divisions, like those separating all other groui)s, 
are not .Aiarply marked, but are more or less insensibly blended 
together by intermediate forms and thus really indicate successive 
and divergent or, iirobably in some cases, convergent stages in their 
jirogTess of specialization. 
The ruLYPL.YCOGNATiiA (ttoXis, many ; a j)late or piece : 
yi'udo5, jaw) embrace the groups Sphi/ntdium and Panctaui, the 
former instituted for the reception of W'liujo cdentala and the latter 
for Ileliv minuthsi Did of Jjca, a form 
apparently synonymous with our 
Hidir pygmifd, all of which agree in 
possessing the remarkable peculiarity 
of segmented or comj)Osite and nearly 
circunioral mandibles, composed of a 
number nf somewhat ([uadrate plates, 
each formed by long chitinous fibres 
extending as a fringe lieyond tlieir 
sharp free edges. The plates, though overlapping each other laterally, 
are distinctly separated medially, all however being slightly con- 
nected together by a delicate membrane. 
At the present time Jlelir pipjDut'it and Vertigo edentula are the 
only species in this country actually known to posssess this remark- 
able type of jaw, llell.r nipestris, which, on strictly testaceological 
gnjunds, is apparently so closely allied to Ilellr piigmird, being a 
decidedly monognathous sjjecies. 
Fig. o 02. — Segmented or primitive 
Polyplacognathoiis mandibles from Helix 
pygniwa I)rap., Jiighly magnified (after 
Schacko). 
