170 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
the limbs of the thorax are all reducible to the same type 
as those of the abdomen, if we suppose that, in the 
posterior five pair, the exopodites are suppressed; and 
that, in all but the last, podobranchiw are superadded. 
Turning to the appendages of the head, the second 
maxilla (fig. 47, C) presents a further modification of the 
disposition of the parts seen in the first maxillipede. 
The coxopodite (cxp) and the basipodite (bp) are still 
thinner and more lamellar, and are subdivided by deep 
fissures which extend from their inner edges. The 
endopodite (en) is very small and undivided. In the 
place of the exopodite and the epipodite there is only 
one great plate, the scaphognathite (sg) which either 
is such an epipodite as that of the first maxillipede 
with its anterior basal process muqh enlarged, or repre- 
sents both the exopodite and the epipodite. In the first 
maxilla (B), the exopodite and thé epipodite have dis- 
appeared, and the endopodite (en) is insignificant and 
unjointed. In the mandibles (A), the representative of 
the protopodite is strong and transversely elongated. Its 
broad inner or oral end presents a semicircular mastica- 
tory surface divided by a deep longitudinal groove into 
two toothed ridges. The one of these follows the con- 
vex anterior or inferior contour of the masticatory surface, 
projects far beyond the other, and is provided with a sharp 
serrated edge; the other (fig. 48, a) gives rise to the straight 
posterior or superior contour of the masticatory surface, 
and is more obtusely tuberculated. In front, the inner 
