122 W. D. Matthew — Antennce and other 



nitely seen and described, are three specimens of Asaphus 

 platycephalus* — all found at different times and places — in 

 which a number of legs are preserved in a fragmentary way, 

 another of the same genus showing a palpusf attached to the 

 hypostome, and a few cases in which a detached leg or antenna 

 was found in company with species of trilobites, and referred 

 to one of them. Other discoveries have been reported from 

 time to time, but not conclusively verified. 



It has, however, been found possible by cutting thin sec- 

 tions of trilobites especially well preserved, 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 success- 

 ful investigation of this kind, extending over several years, 

 and based on the study of over 2000 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 involved : 



1. A ventral membrane over the under side of the body, 

 with hardened arches across each segment, bearing the ap- 

 pendages. 



2. Cephalic limbs. There were four pairs of these, the last 

 of which was larger and expanded at the terminal joint into a 

 swimming organ. The bases of these limbs were manducatory 

 in their action as in the modern King-crabs. 



3. Thoracic and 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 branchiae, which were in the form of narrow spiral 

 ribbons. The legs, like those of the head, were generally com- 

 posed of six joints, which were more or less conical, the basal 

 end smallest. Other forms of gills were also met with, viz: a 

 straight uncoiled ribbon in immature specimens, and a radiat- 

 ing leafy form found sometimes in the anterior part of the 

 thorax. There is no mention of cephalic gills. 



4. JSTo trace of any aniennal 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, with very limited swimming powers, when mature. 



*Quar. Jour. Geol. Soc, 1870, p. 479; Cincinnati Jour. Nat. Hist, vol. vi, 1883. 



fQuar. Jour Geol. Soc, 1870, p. 186. 



JBull. No. VIII, Harvard Museum of Xat. Hist. Mr. Walcott gives very com- 

 plete references and history up to that date, which I have not thought necessary 

 to repeat. 



