MARINE CRUSTACEANS. 693 
shows a tendency to be flattened. The Nephropsidea differ from them in all these respects. 
Between the Eryonidea and the Scyllaridea the most striking difference lies in the fact 
that in the former all the legs are chelate and in the latter none are so. The Eryonidea 
are the more primitive in keeping the antennal scale, the joint between the ischiopodite 
and meropodite, and the chelae on the first three legs. 

Fie. 126. Limbs of Avius acanthus; a, second maxilla, b. first maxilliped, c. abdominal limb of the third pair. 
1. gnathobase or first lobe of coxopodite, 2. endopodite, 3. appendix interna, 4. lobes of basipodite. 
In considering the second set of groups, we have first to settle whether to look upon them 
as of common descent and treat them as a whole, or whether we must find a separate point 
of origin for each of them from among the Macrura. Now it may be allowed at the outset 
that there is but one character that every member of them shares with the rest. In the 
macrurous groups the first three pairs of legs are all alike, adapted either to seizing or to 
crawling—chelate, that is, or simple. But in the modified groups we are now dealing with 
the third pair is never adapted to the same end as the first. In the overwhelming majority 
the first pair are chelate or subchelate’ and the third used for walking, and in the Hippidae, 
where the first are simple and straight, the third are hooked and flattened for burrowing or 
swimming. Nor is this point an unimportant one, for the likeness of the first three legs 
is a heritage from the penaeid prawns, and as such is of considerable value. Moreover it 
is possible to find a number of characters which, while they are not seen in every member 
of the non-macrurous groups, are yet common in each of them, and do not occur elsewhere. 
These are: (1) a large gnathobase, or inner coxopoditic lobe, on the second maxilla (Fig. 126 a), 
(2) the shape of the first maxilhped, which has a broad but shallow basipoditic lobe, whose 
inner margin is in the same line with that of the coxopodite*®, and a broad basal half to 
the exopodite (Fig. 126 6), (8) the flagella of the maxillipeds turned inwards and curved 
forwards at their ends, (4) reduction in various ways of the abdomen, (5) the presence of 
backward prolongations of the longitudinal lines of the carapace*. Since, however, numbers 
(8) to (5) are less strongly marked in the more primitive forms of some of the groups, it 
is doubtful whether much stress can be laid upon them. 
Taking it for granted, then, that the non-macrurous Decapods had a common, if remote, 
ancestor, it remains to be seen where that ancestor must be placed with regard to the 
1 Gebicula seems at first sight to be an exception to this, ‘‘ thumb” of the allied Upogebia. 
but there is on the propodite a strong tooth near the end * Found also in at least one species of Nephropsis. 
which seems almost certain to be used in the same way as the 3 See above, footnote to p. 691. 
G. Il. 89 
