J^ossils of the Utica Slate, 



25 



in the metamorphoses of the living Phyllopodes (according to the 

 description of Burmeister cited above), are forever effaced for us." ^ 

 " The Savant Professor of Halle (Burmeister) still teaches us that 

 at the earliest age all the modern Phyllopodes — without exception — 

 are naked, that is, deprived of all crustacean envelope, according 

 to general analogies demonstrated by this savant, between this 

 tribe and the trilobites, we can admit as very reasonable that at 

 least a part of these ancient crustaceans were born in the same state 

 of nudity." 



Four orders of variatious are characterized in the metamorphoses 

 of the trilobites in which he has observed evidence of progressive 

 development. 



"They are as follows: 



1st order. 



2d order. 



3d order. 



4th order. 



iHead predominating, incomplete. 

 Thorax nothing or rudimentary. 

 Pygidium nothing. 



{Head distinct, incomplete. 

 Thorax nothing. 

 Pygidium distinct, incomplete. 



Type 



Sao hirsuta. 



Type 



( Head 

 \ Thor 



complete, 

 horax distinct, incomplete. 

 Pygidium distinct, incomplete. 



iHead complete. 

 Thorax complete. 

 Pygidium distinct incomplete. 



|- Trinucleus ornatus and all 

 ) the Agnostus. 



u 



Type 

 rethusina Konincki. 



Type 



Dalmanites Hausmanni. 



" The two last sections are provisional and ought sooner or later 

 to be merged into the one, or other of the two first." 



We find Triarthrus Becki placed doubtfully at the end of the 

 3d order on the description given by Prof. Hall. We cannot re- 

 move it from that position, as the youngest stage we have shows one 

 segment in the thorax, but from the changes that occur in its de- 

 velopment, we cannot avoid the conclusion that with more complete 

 material it will be removed to the 2d order, as the smallest individual 

 is one millimetre in length and the head and pygidium are strongly 

 lobed and well developed. 



That the earlier degrees of development of Sao hirsuta^ the type 



* In the earlier degree of development of the trilobite, and for all preserved 

 in the sediments of the character of those in Bohemia, this undoubtedly is true. 

 Since the discovery ^ of the delicate spiral branchial apparatus of the trilobite, 

 preserved in the fine grained limestones of the Trenton group, it is to be hoped 

 that the time is not far distant when even some of the later changes in the pro- 

 gressive development of the so called soft parts may be discovered and give to 

 the biologist more of the life history of this remarkable crustacean. We have a 

 few facts sustaining this view which will be given in a paper on the anatomy 

 and development of some trilobites of the Trenton limestone. 



' 3l8t Report N. Y. State Museum of Natural History, 1878, and Preliminary Notice, 1877. 

 Trans, x.'] 4 



