MARRELI.A SPLENDENS. I I 5 



In Isopoda the antenna? 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 ligioidcs Carpenter and Swain (Proc. Royal 

 Irish Acad., vol. 27, sect. B, 1908, p. 63, fig. I ), 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, Oxyurofoda would 

 be a very primitive isopod in 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 setae. The anal segment consists of a single plate. 



