from the Lower Helderoerg Group. 143 



The first American larval trilobites were discovered and 

 described by S. W. Ford.* They were specimens of Ellipto- 

 cephala {Olenellus) asaphoides Emmons, from near Troy, N. Y., 

 and together with the larvse of Sao hirsuta were the most 

 elementary forms known. 



Next, C. D. Walcott, in 1879,f published a fine series illus- 

 trating the metamorphoses of Triarthrus JBecki Green, from 

 the Utica Slate. All the stages were shown, from an indi- 

 vidual with one thoracic segment to mature specimens having 

 thirteen to sixteen segments. 



G. F. Matthew, in" 1887 and 18894 carefully described and 

 illustrated embryonic and immature forms of Ptychoparia 

 Linnarssoni Brogger, Liostraous ouangondianus Hartt, and 

 Solenopleura JRobbi Hartt, from the slates at St. John, New 

 Brunswick. The youngest forms were quite as primitive as 

 the, Sao of Barrande and Elliptocephala of Ford, so that the 

 larvse of at least five genera have been described, representing 

 the same early stage. 



The orders of development recognized by Barrande are : 



( Head predominating, incomplete. ^ Types. 



I. < Thorax nothing or rudimentary. > Sao hirsuta. 



( Pygidium nothing. ) 



( Head distinct, incomplete. ) Trinucleus ornatus and 



II. < Thorax nothing. f- all Agnosias. 

 ( Pygidium distinct, incomplete. 

 ( Head complete. 



III. } Thorax distinct, incomplete. j- Arethusina Konincki. 

 ( Pygidium distinct, incomplete. 

 ( Head complete. J 



IV. } Thorax complete. >- Dalmanites Hausmanni. 

 ( Pygidium distinct, incomplete. ) 



The third and fourth orders are probably later stages of the 

 first and second, and it now seems that the second order will 

 be found to have an earlier stage equivalent in structure to the 

 first. 



The larvse noticed in the present paper are of peculiar 

 interest on account of their belonging to highly ornamented 

 and specialized genera. Moreover, some deductions can now 

 be made concerning the permanence and uniformity of the 

 stages of metamorphoses, as well as the acceleration of specific 

 characters so that they appear in the larval state. 



*This Journal, vol. xiii, p. 265, 1877. 



f Transactions of the Albany Institute, vol. x. 



X Illustrations of the Fauna of the St. John Group, No. IV, Trans. Roy. Soc. 

 Canada, 1887. Sur le Developpment des premiers Trilobites, An. Soc. Roy. 

 Macol. Belgique, xxiii. 1889. 



Am. Jour. Sol— Third Series, Vol. XLVI, No. 272.— August, 1893. 

 10 



