5. FURTHER OBSERVATIONS ON THE VEN- 

 TRAL STRUCTURE OF TRIARTHRUS * 



(Plates VII and VIII) 



In previous papers on the ventral structure of Triarthrus, 

 the anterior antenna?, thoracic legs, and appendages of the 

 pygidium have been described. f There yet remain for inves- 

 tigation the appendages of the head and additional details of 

 other parts of the animal. These characters have not been 

 easily obtained on account of the labor of removing the rock 

 from such delicate structures. Moreover, but few specimens 

 are in the proper position or condition to 3'ield satisfactory 

 results. The appendages of the head either suffered greater 

 decomposition than those of the thorax, before mineraliza- 

 tion, or were so tenuous as to be easily obliterated, and are 

 now seldom sufficiently well preserved for study. Further, 

 the number and compact arrangement of such complicated 

 organs, even when present, make it difficult to trace their 

 precise form. A similar difficulty would be experienced 

 were one to endeavor to describe the appendages of Apus by 

 examining the ventral side without cutting away some of 

 the limbs or at least unfolding or bending them around. 



* American Geologist, XV, 91-100, pis. iv and v, 1895. 



t W. I). Matthew. — On Antennaj and otlier appendages of Triarthrus 

 Beckii. A'^. Y. Academi/ of Sciences, May, 1893 ; xlmcr. Jour. Sci., August, 1893. 



C. D. Walcott. — Note on some Appendages of tlie Trilobites. Proc. Biol. 

 Sac. Washiwjton, March, 1894. 



C. E. Beccher. — On the Thoracic Legs of Triarthrus. Amer. Jour. Sci., 

 December, 1893. 



On the mode of Occurrence, and the Structure and Development of 



Triarthrus Becki. American Geologist, January, 1894. 



The Appendages of the Pygidium of Triarthrus. Amer. Jour. Sci., 



April, 1894. 



