434 ZOOLOGY. 



Walcott completed the proof of the existence of. true legs iu Trilobites 

 of the genera Calymeue and Ceraurus. Thus, too, the correctness of an 

 observation made about six years before (in 1870) was confirmed, for 

 Mr. E. Billings, the palaeontologist of Canada, then announced the dis- 

 covery of traces of legs. The accuracy or relevancy of his observations 

 was generally denied, however, and certainly they needed confirmation. 

 Mr. Walcott, continuing his studies, in 1881 published the results of his 

 researches in a memoir on " The Trilobite; new and old evidence relating 

 to its organization." He has done his work well, and by means of 

 numerous sections, longitudinal as well as transverse, has traced the 

 course and structure of the appendages, and at least conclusively estab- 

 lished the existence of legs homologous with those of the King crab 

 and Eurypterids. But he further claims that such appendages were 

 repeated on each segment, and that in addition to the homologues of 

 the five pairs of legs whose basal joints i)erform the office of manduca- 

 tion, there are "numerous thoracico-abdominal appendages." If this 

 statement is the true expression of the facts, it is evident that while 

 the relations of the Trilobites with the typical Merostomes are estab- 

 lished, the type of structure is in the highest degree peculiar. So 

 remarkable would be the deviation from the standard that probably 

 many may feel disposed to await further evidence and suspend opinion, 

 lest the observed facts may be susceptible of some other interpretation. 

 That which will probably provoke most skepticism is the attribute to 

 the pygidium of jilural pairs of limbs similar to the others, and indeed 

 Mr. Walcott himself admits that as to the character of those ai^pend- 

 ages "the evidence is not all that could be desired" (p. 204). On the 

 other hand, it may be added that the well-known variability, according 

 to group as well as to individuals, of the number of segments prepares 

 us to expect, or at least not to be unduly surprised at, some remarka- 

 ble deviation from the typical mode of segmentation and appeudicidar 

 api)aratus. We may, therefore, concede that the limbs are existent in 

 increased number, but must hold in abeyance confession of belief in 

 their extension as such, and unmodified, to the extreme end of the 

 pygidium. 



The facts being admitted, even with reservation as to details, little 

 fault can be found with Mr. Walcott's systematic conclusions. He rec- 

 ognizes as constituents of a peculiar "class," under the name "Pceci- 

 lopoda," the typical "Merostomata" (of which he makes a sub-class 

 with two "orders" — the Xiphosura and Eurypterida) and the Trilo- 

 bites, which, at the same time, represent a "sub-class Palaeadne" and an 

 "order Trilobita." He formulates the results of his comparative re- 

 searches on the structure of the last in his diagnosis of the "sub-class" 

 as " Poeciloi)ods, with numerous thoracico-abdominal appeudages, eyes 

 compound (when developed), ocelli unknown," while the "order" is 

 distinguished by a "mouth furnished with a large hypostoma and four 

 pairs (as far as known) of appeudages j thoracic segments, 2 — 26, bear- 



