TRE}NTON CONGLOMERATE OF RYSBDORPH HILL 53 



pygidium of A. r o s t r a t u s Sars. These authors describe 

 the axis of the pygidium of that species as faintly seg- 

 mented and with a row of tubercles on each side. As the 

 English species is in all features very closely related 

 to that from the Trenton, and the latter never shows any 

 tubercles on the axis of the external crusty but only on the 

 casts, the writer feels sure that Nicholson and Etheridge had 

 only casts of the pygidium, and based their conclusions on these. 

 These authors, how^ever, figure on pi. 10, fig. 20, of the same fasci- 

 culus a pygidium; of an A s a p h u s sp. ind., the slender axi;^ 

 of which, though much weathered, exhibits what appears to be 

 a series of depressions on each side. Here the weathering has 

 evidently reached the depressions in the crust, to which the above 

 mentioned tubercles correspond. Similar depressions have 

 before been observed by Salter^ of A s a p h u s t y r a n n u s , 

 and considered by him to be internal glands, a view which evi- 

 dently is also held by Nicholson and Etheridge. The writer has 

 also obtained a small pygidium oflsotelus maximus from 

 the Rysedorph hill conglomerate, which by the removal of the 

 crust exhibits a series of eight pairs of tubercles which are some- 

 w^hat obliquely directed backward (pi. 4, fig. 1). 



The only observation of w^hich the writer is aware which 

 has been made on such pits in this country is that by 

 Hall and Clarke^ on a pygidium of P r o e t u s f o 1 1 i - 

 c e p s of the Onondaga limestone. In that specimen two 

 paired series of alternating elevations can be observed; 

 all of the elevations are however obliquely elongate, the 

 inner series consists of the larger elevations, and the inner- 

 most elevations lie apparently directly on the axial line and 

 are not paired. The same authors observed similar, though not 

 so distinctly preserved markings on the internal cast of a pygi- 

 dium of Proetus crassimarginatus from the Onon- 

 daga limestone of Ohio^ and the paired marginal impressions 



» aionograph. pi. 22, fig. 9. 



2 Pal. N. Y. 1888. 7:101, pi. 29, fig. 1. 



=" Pal. N. Y. 1888. v. 7, pi. 25, flg.'S. 



