No. 10. — The Trilobite: New and Old Evidence relating to its 

 Organization. By C. D. Walcott. 



INTRODUCTION. 



This publication terminates, for the present, an investigation that 

 has occupied much time and attention during the past seven years. 



In the month of October, 1873, the attention of Professor Louis Agas- 

 siz was called to certain markings on the inner side of the pleur£e of a 

 specimen of Asaphiis platycephalus, " Panderian Organs." He con- 

 sidered them as proving the existence of true crustacean legs. {Amer. 

 Nat., VIL 741, 1873.) With his characteristic liberality Prof. Agassiz 

 offered facilities for study, and strongly urged that an attempt should 

 be made to discover the ventral surface of the animal and the character 

 of the attached appendages. It affords me pleasure to state here that 

 whatever there may be of value in this contribution to our knowledge 

 of the subject is owing largely to the spirit of investigation that he 

 awoke and which has carried forward the work under many adverse 

 circumstances long since his death. 



The prosecution of the investigation during the year 1874 gave 

 the material from which the notes on the inferior or ventral surface of 

 the dorsal shell of Ceraiirus pleurexanthemus were written. Judging 

 from tlie dorsal shell alone the views of Burmeister were given as best 

 explaining the facts then known.* 



The succeeding year thin sections of Trilobites were cut from both 

 Lower and Upper Silurian rocks. In the upper portion of the Trenton 

 limestone at Trenton Falls, N. Y., a thin layer of dark, bluish-graj'-, 

 fine-grained, partially impure limestone was found, that contained 

 many very perfectly preserved trilobitic remains. On examination of 

 these by cutting sections, it was ascertained that other parts of the 

 animal besides the dorsal shell and hypostoma were present. Specimens 

 from all other localities and formations failed to afford more than the 

 strong dorsal shell and hypostoma. This fact once established led to 

 the extended working of the prolific stratum. The soil and rock to a 

 depth of nine feet was removed, over a large area, to obtain the fos- 

 sils scattered through the thin layer of limestone. From this area 



* Aim. Lyceum Nat. Hist, of New York, XL pp. 155-162, 1876. 



VOL. VIII. — NO. 10. 



