206 XEJW YORK STATE MUSEUM 



All specimens figured on plates 1-7 are from the iQwer Trenton eon- 

 glomerate of Rysedorph bill, lleusselaer eo. N. Y., if not otherwise stated. 



PLATE I 



(All originals in stale museum) 



Bolboporites americanus Billings 



p. 11 



Fig. 



1 A somewhat weathered specimen, x 3 

 Pebble of group 3 



Stomatopora inflata Hall sp. 

 p. 12 



2 A somewhat turgid variety, x 10 



3 A normal form, x 10 

 Pebbles of group 5 



% Siphonotreta minnesotensis Hall & Clarke 



p. u 



4 A pedicle valve, x 4 



5 Lateral view of same, x 4 

 Pebble of group 5 



Plectambonites sericeus Sowerby var. asper James 



p. 18 



6 An adult specimen, showing well developed cardinal corruga- 



tions. X 2 



7 A younger specimen with still stronger corrugations, x 2 

 Pebbles of groups 6 and 7 



^!Br 



Plectambonites pisum sp. nor. 

 p. 19 



8 A j)edicle valve with i"elatively long cardinal line, x 2 



9 Lateral view of same, x 2 



10 A less tumid pedicle valve, x 2 



11 Posterior view of same, x 2 



12 Pedicle valve of normal dimensions, x 2 



