C. E. Beecher — Thoracic Legs of Triarihrus. 467 



Art. LXII. — On the Thoracic Legs of Triarihrus ; by 

 C. E. Beecher.* 



The first conclusive evidence of the presence of legs and 

 gills in the Trilobita was given in 1876 by C. D. Walcott,f 

 who by means of translucent sections was enabled to demon- 

 strate their existence in four genera, and to indicate several 

 important points in their structure. The genera shown to 

 possess legs were Ceraurus, Calymene, Asaphus, and Acid- 

 aspis. Various announcements of the discovery of the legs 

 of trilobites had been made previously, but, with the excep- 

 tion of a specimen described by Billings in 1870, these proved 

 to be erroneous determinations. Walcott has since made 

 further studies, and brought together his conclusions in a paper 

 published in 18814 Until 1893, no additions of note have 

 been made beyond those confirming the fact that these animals 

 possessed jointed legs. The specimens investigated by Walcott 

 consisted chiefly of coiled individuals in which the body cavity 

 was filled with calcite and clayey matter. There was no actual 

 mineralization of the appendages, but the fillings of the cavi- 

 ties left by the decay of these organs served to show their 

 general form. On account of their enrolled condition, con- 

 siderable difficulty was experienced, and there was also a liabil- 

 ity to error in attempting to determine the precise characters 

 of the organs observed. Notwithstanding these drawbacks, 

 some of the main points in the ventral anatomy of these ani- 

 mals were established. 



The occurrence of Triarihrus Becki Green, in an excep- 

 tionally perfect state of preservation, from the Utica Slate 

 near Rome, N. Y., has already been noticed in this Journal. § 

 In their present condition, the specimens contain very little 

 calcareous matter, and nearly the entire calcareous and chiti- 

 nous portions of the animal are represented by a thin film of 

 iron pyrite. To this kind of fossilization is doubtless due the 

 preservation of delicate organs and structures which otherwise 

 would have been destroyed. For, as is well known, pyrite 

 may replace such organic tissues as chitine or even soft dermal 

 structures, the change occurring by the slow decomposition of 



♦Abstract of a paper "On the Structure and Development of Trilobites'' 

 read before the National Academy of Sciences, November 8, 1893. 



f Preliminary Notice of the Discovery of the remains of natatory and bran- 

 chial appendages of Trilobites. 28th Ann. Rept. N. Y. State Museum of Natural 

 History, published in advance. December, 1876. 



% The Trilobite : new and old evidence relating to its organization. Bulletin of 

 the Museum of Comparative Zoology, vol. viii, No. 10, 1881. 



§ Vol. xlvi, pp. 121-125, 378-379, August and November, 1893. 



