238 TEAKSACTIONS OF THE [MaY 22 



in a fragmentarj' way ; another of the same genus, showing a 

 jDalpus attached to the hypostome, and a few cases where a 

 detached leg or antenna was found in company with species of 

 trilohites, 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 especiall}^ well preserved in limestone, to determine 

 the nature and position of the organs of the vander 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, Geraurus 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 limbx. There are four pairs of these, the last of 

 which is larger and expanded at the tei'minal joint into a swim- 

 ming organ. The bases of these limbs were manducatory in 

 their action. 



3. Thoracic abdominal appendage!^. 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 

 branchiae 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 anieiinal )<y.-<fem 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 Avith 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. 

 The}' occur in the Hudson River shales* near Rome, N. Y. A 

 number of the best specimens are now in the museum of 



* Though referred to the Hudson River bv Mr. Valiant, Prof. Whitfield is 

 disposed, on pala'oatological grounds, to consider them as Utica shales. 



