xiv INTRODUCTION. 
incisive margin, the two extremities or horns of a cres- 
cent. The second, or articulated process, is situated 
between the two, but somewhat nearer the anterior 
margin. It appears to be able to assist in carrying the 
food from the one point to the other, from the biting to 
the grinding surfaces, between which and the molar 
tubercle are frequently a row of strong and curved spines 
that facilitate the process. 
The mandibles are moved by powerful muscles attached 
to the inner surface of the dorsal part of the cephalon, 
corresponding with the homological parts that are 
attached to the inner dorsal surface of the carapace 
of the higher Crustacea. 
The surface of the molar tubercle is granulated with 
rows of minute denticles that are only visible under a 
strong magnifying power. In some species, a long and 
slender ciliated filament is appended to the margin of the 
tubercle that may be associated with the sense of taste. 
The mandibles are no exception to the fact that all 
appendages are but modified legs. In all Crustacea, we 
think that it can readily be demonstrated that the man- 
dible consists of the first three joints being closely anchy- 
losed. The small appendage, that generally consists of 
three freely articulated joints, represents the fourth, fifth, 
and sixth joints; the seventh, or dactylos, being seldom 
present. An homological examination of the genera 
Nebalia and Pontia, with Homarus, together with the 
homotypical parts in other appendages in the same ani- 
mals, we think will readily confirm this opinion. The 
small three-jointed appendage to the mandible is wanting 
in but few genera, excepting in the terrestrial Jsopoda and 
Amphipoda. In aquatic species it is, with few exceptions, 
always present, and appears to be of efficient use in 
directing floating material towards the mouth. 
