\ 

 C. D. WalcoU — Appendages of Trilohites. 249 



coiled spiral. The outer side is a cylindrical, jointed, stem-like rim 

 that is attached to the inner side, a narrow distinctly impressed line 

 separating the two, except at the somewhat flattened tip where they 

 merge into each other. On the outer or upper surface of the outer 

 side numerous crenulations occur that extend into long set^, n, 

 Fig. 2 ; h, h, Fig. 1. Dr. Beecher considers the expedite as a 

 swimming organ ; but from the manifest branchial character of 

 the exopodite and attached epipodite in Calymene (Fig. 7), it seems 

 probable that this exopodite of Triarthrus served largely as a gill, 

 and that the animal used the broad proximal joints of the posterior 

 limbs of the thorax as its principal propulsion in swimming. The 

 exopodite of Triarthrus looks like a consolidated exopodite and 

 epipodite, very much as though these two organs, as they occur in 

 Calymene, were merged into one. 



Several specimens illustrate appendages beneath the pygidium. 

 Some have the broad proximal joints, d, Fig. 1, while others show 

 the outer rim of the exopodite, c, Fig. 1. The material I have 

 seen indicates very little difference between the appendages of the 

 posterior half of the thorax and the pygidium, except that those 

 of the latter are less developed in size and details. 



Mr. Matthew suspected the presence of a flap, formed by the 

 anchylosing of the appendages beneath the pygidium. From the 

 appearance of a similar structure, where the limbs are matted 

 together along the side of the thorax, this tentative view is 

 received with doubt. More perfect material may show distinctions 

 not recognizable at present. 



If future investigations prove, as it now seems probable, that 

 the modified swimming joints of the endopodite are attached to 

 ten or more of the thoracic segments, the anterior eight segments 

 can be grouped together as the typical thorax, and the remaining 

 segments of the body as the abdomen. 



Mr. Matthew suggests that the homology between Triarthrus and 

 LimuJus may not be as close as between Limuliis, Calymene and 

 Ceraurus. This is true from what we now know of Triarthrus, but, 

 if a sixth pair of cephalic limbs should be discovei-ed in Triarthrus 

 the resemblance would be strengthened. Triarthrus does not differ 

 from Ceraurus and Calymene more than would be anticipated in such 

 unlike genera. Triarthrus is essentially a "Primordial" type that 

 has continued until Upper Ordovician time. It represents a large 

 group of Cambrian Trilobites, whilst Calymene and Asaphns repre- 

 sent the more highly developed Ordovician and Silurian forms. 



Dr. Lang held the view ^ that if a fifth pair of cephalic limbs were 

 found, comparable to the anterior antennae "Trilobites might then 

 be regarded as original Entomostraca, to be derived from the same 

 racial form as the Phyllopoda." He says, further," " Xiphosura, 

 Hemiaspidre, and Gigantostraca are themselves again perhaps 

 racially connected with the Trilobites. In any case, however, in 

 the present state of science, it seems probable that all these groups 

 are only connected at their roots with the Crustacea." 



1 Text Book of Comparative Anatomy, English Ed. 1891, p. 415. 



2 £qq^ qh^ p_ 421. 



