J. D. Dana on the Classification of Crustacea. 5 
"The most perfect state of it is seen in the Maia group, (the tri- 
angular erabs,) in which the bases of the antenns and eyes are 
crowded into the narrowest possible compass, and the mouth 
organs are well compacted within the buccal area, and the legs 
and whole system have the highest completeness. 
The form of the body of a Maia is a somewhat flattened ovoid, 
narrowest in front; and the middle point between the mouth 
and the second antenns, which we call the potential centre of - 
the animal, is situated near the front, say about half an inch 
from the front outline (excluding the beak), supposing the ceph- 
» alothorax three inches long. "We may call the part anterior to 
this centre, A ; the part posterior, B; and the length of the for- 
mer, measured on the axis, «; of the latter, b. 'lhese parts 
may be viewed, as regards development, as potentially equal; 
and yet the anterior, A, is six times shorter and as much nar- 
rower and lower than the following. It would not, therefore, 
be far out of the way to say, in mathematical language, that the 
funetional importanee of the two parts varies inversely as the 
cubie contents of the parts. 
We pass now to the degradations from this, the highest type. 
"These degradations are seen— 
J'irst, 3n à, widening of the space between the antenne. 
Second, in & slight enlargement of the outer maxillipeds, so 
that they do not fit snugly over the buccal area. 
Third, 1n. an elongation of the antennz. 
'T'hese are all evidences of a slight relaxing of the concentrat- 
ing element. 'lhe first, marks the transition of the Maia group 
to the Parthenopids, and thence to the Cancrids.  'lhe second, 
carries the grade a step lower, to species of the old genus Cancer, 
also to the swimming crabs and the Corystoids; and the third, 
marks off the Corystoids as the lowest of the true Brachyura. 
While there are such marks of degradation exhibited through 
the growth or elongation of parts, there is also a mark, equally 
significant, in the obsolescence of the posterior thoracic legs, a 
peculiarity of many Grapsoids. In the Maioids, the species are 
well balanced; the type is perfect in its development; the sus- 
taining of the central functions allows of the full and complete 
growth of all the other parts. But the diminution of force 
may not only be attended with a loosening of the cephalie hold 
on the remoter of the cephalie organs, but also, in a failure in 
the production of the posterior organs of the body, or those on 
the outer limits of the system: and this is what happens in 
many Grapsoids. 'l'he swimming form of the legs in Lupa and 
allied species is à. similar mark of inferiority. 
Besides the above evidences of degradation, there are still 
others in the Brachyural structure, whieh act conjointly with 
SECOND SERIES, VOL. XXII, NO. 644.—JULY, $56. | 2 | 
