MARRELLA SPLENDENS. I I 5 
In Isopoda the antennae are practically uniramous sensory organs. The second cephalic 
appendages of trilobites are capable of such development through reduction of the exopodite. 
In the Isopoda the coxopodites are usually fused with the body, remaining as free, 
movably articulated segments only in a part of the thoracic legs of one suborder, the Asellota. 
Endobases are entirely absent. This is of course entirely unlike the condition in Trilobita, 
but a probable modification. 
In Isopoda there is a distinct grouping of the appendages, with specialization of func- 
tion. The trilobites show a beginning of tagmata, and such development would be expected 
if evolution were progressive. 
In both groups, development from the embryo is direct. Rudiments of exopodites of 
thoracic legs have been seen in the young of one genus. 
The oldest known isopod is Oxyuropoda ligioides Carpenter and Swain (Proc. Royal 
Irish Acad., vol. 27, sect. B, 1908, p. 63, fig. 1 ), found in the Upper Devonian of County 
Kilkenny, Ireland. The appendages are not known, but the test is in some ways like that of 
a trilobite. The thorax, abdomen, and pygidium are especially like those of certain trilo- 
bites, and there is no greater differentiation between thorax and abdomen than there is be- 
tween the regions before and behind the fifteenth segment of a Pcedeumias or Mesonacis. 
The anal segment is directly comparable to the pygidium of a Ccraurus, the stiff unseg- 
mented uropods being like the great lateral spines of that genus. 
The interpretation of the head offered by Carpenter and Swain is very difficult to under- 
stand, as their description and figure do not seem to agree. What they consider the first 
thoracic segment (fused with the head) seems to me to be the posterior part of the cephalon, 
and it shows at the back a narrow transverse area which is at least analogous to the nuchal 
segment of the trilobite. If this interpretation can be sustained, Oxyuropoda would 
be a very primitive isopod i.n which the first thoracic segment (second of Carpenter and 
Swain) is still free. According to the interpretation of the original authors, the species is 
more specialized than recent Isopoda, as they claim that two thoracic segments are fused 
in the head. The second interpretation was perhaps made on the basis of the number of 
segments (nineteen) in a recent isopod. 
MARRELLA SPLENDENS WALCOTT. 
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 192, pis. 25, 26. 
Among the most wonderful of the specimens described by Doctor Walcott is the "lace 
crab." While the systematic position was not satisfactorily determined by the describer, 
it has been- aptly compared to a trilobite. The great nuchal and genal spines and the large 
marginal sessile eyes, coupled with the almost total lack of thoracic and abdominal test, give 
it a bizarre appearance which may obscure its real relationships. 
The cephalon appears to bear five pairs of appendages, antennules, and antennae, both 
tactile organs with numerous short segments, mandibles, and first and second maxillae. The 
last three pairs are elongate, very spinose limbs, of peculiar appearance. They seem to have 
seven segments, but are not well preserved. These organs are attached near the posterior 
end of the labrum. 
There are twenty-four pairs of biramous thoracic appendages, which lack endobases. 
The endopodites are long and slender, with numerous spines; the exopodites have narrow, 
thin shafts, with long, forward pointed setre. The anal segment consists of a single plate. 
