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24. DIFFERENT KINDS OF BILLS. 
vertebrate animals. All bills, thus similarly constituted, have been 
divided * into 
§ 45. Four Cuassms, representing as many ways in which the two man- 
dibles close upon each other at the end. 1. The epignathous (Gr. epi, upon, 
gnathos, jaw) way, plan, or type, in which the upper mandible is longer 
than the under, and its tip is evidently bent down over the tip of the lower. 
2. The hypognathous (Gr. hypo, under), in which the lower mandible is 
longer than the other. 3. The paragnathous (Gr. para, at or by), in which 
both are of about equal length, and neither is evidently bent over the other. 
4. The metagnathous (Gr. meta, with, beside, etc.), in which the points of 
the mandibles cross each other. The second and fourth of these are ex- 
tremely rare; they are exemplified, respectively, by the skimmer and the 
cross-bill (genera 295 and 60). The first is common, occurring through- 
out the birds of prey, the parrots, and among the petrels, gulls, etc., etc. 
The great majority of birds exhibit the third; and among them, there is 
such evident gradation into epignathism, that it is necessary to restrict the 
latter to its complete development, exhibited in the intermaxillary bone di- 
vested of its horny sheath, which often, as among flycatchers, etc., forms a 
little overhanging point, but does not constitute epignathism. These classes, 
it should be added, though always applicable, and very convenient in de- 
scriptions, are purely arbitrary, that is, they by no means correspond to 
any four primary groups of birds, but on the contrary, usually only mark 
families and the subdivisions of families; and the four types may be seen 
in contiguous genera. The general shape of the bill has also furnished 
§ 46. Orner Crasses, for many years used as a large basis for ornitho- 
logical classification ; but which the progress of the science has shown to be 
merely as convenient as, and only less arbitrary than, the foregoing. The 
principal of these are represented by the following types :— A, among land 
birds. 1. The jisstrostral, or cleft, in which the bill is small, shoré, and 
with a very large gap running down the side of the head, as in the swallow, 
chimney-swift, whippoorwill. 2. The éenucrostral, or slender, in which the 
bill is small, Jong, and with a short cleft; as in the humming-bird, creeper, 
nuthatch. 38. The dentirostral, or toothed, in which, with a various gen- 
eral shape, there is present a nick, tooth, or evident lobe in the opposed 
edges of one or both mandibles near the end; as in the shrike, vireo, and 
some wrens, thrushes and warblers. 4. The conirostral, or conical, suffi- 
ciently defined by its name, and illustrated by the great finch family and 
some allied ones. B, among water birds. 5. The longirostral, or long, an 
aquatic style of the tenuirostral, best exhibited in the great snipe family. 6. 
The pressirostral, or the compact, illustrated by the plovers, etc., and quite 
likely analogous to the conirostral. 7. The culétrirostral, cutting, perhaps 
analogous to the dentirostral, exemplified in the heron group. None of 
these are now used to express natural groups, in strict definitions; all are. 
* By the writer: Proc. Acad. Nat, Sci. Phila., Dec. 1869, p. 213. 
