238 TRANSACTIONS OF THE [May 22 
in a fragmentary way ; another of the same genus, showing a 
palpus attached to the hypostome, and a few cases where a 
detached leg or antenna was found in company with species of 
trilobites, and referred to one of them. Other discoveries have 
been reported at various times, but not conclusively verified. 
It has, however, been found possible by cutting thin sections 
of trilobites especially well preserved in limestone, to determine 
the nature and position of the organs of the under side. Mr. 
C. D, Walcott in 1881, published the results of a very successful 
investigation of this kind, extending over several years, and 
based on the study of over 2,000 thin sections. His article has 
given us a very complete knowledge of the organization, at 
least of the two genera, Ceraurus and Calymene, which were 
the subjects of his study. 
The structure as thus determined was : 
1. A ventral membrane over the under side of the body, with 
hardened arches across each segment bearing the appendages. 
2. Cephalic limbs. There are four pairs of these, the last of 
which is larger and expanded at the terminal joint into a swim- 
ming organ. The bases of these hmbs were manducatory in 
their action. 
3. Thoracic abdominal appendages. One pair of legs was found 
to be attached to each segment of the thorax and pygidium. 
On the basal joint of each was a small epipodite, and two 
branchize which were in the form of narrow spiral ribbons. The 
legs, like those of the head, were generally composed of six 
joints, which were more or less conical, the basal end smallest. 
Other forms of gills were also found, a ’ straight uncoiled ribbon 
in immature specimens, and a radiating leafy form confined to 
the anterior part of the thorax. There is no mention of cephalic 
gills. 
4. No trace of an anfennal system was found. 
Mr. Walcott concludes that the trilobites were more nearly 
allied to the Limulids than to any other living form, and should 
be classed with them and the Eurypterids, but as a separate sub- 
class. As to their habits, he concludes that they probably were 
free swimming only when young, and crawled around on the 
bottom when mature. 
Mr. W.S. Valiant has found a very considerable number of 
specimens of Triarthrus Beckii, in which the organs of the 
under side are attached to the body and fairly well preserved. 
They occur in the Hudson River shales* near Rome, N. Y. A 
number of the best specimens are now in the museum of 
* Though es ae to the Hudson River by Mr. Valiant, Prof. Whitfield is 
disposed, on paleontological grounds, to consider them as Utica shales. 
