GENERA STREPTOEHYNCHUS AND STEOPHODONTA. 241 



II. OBSERVATIONS ON THE GENUS STREPTORIIYNCIIUS- 

 III. ON THE GENUS STROPHODONTA. 



These papers will be deferred till the next Report, on account of the 

 non-completion of the illustrations intended to accompany them, and 

 without which the discussions would be very unsatisfactory. 



The result of a critical examination of the species known in the higher 

 rocks of New- York has resulted in referring' to Srtoyhomena chemungensis 

 (Conrad) the following forms, which are apparently only varieties of 

 this very variable species : Strophomena hifurcata (Hall), S. arctostriata 

 (Hall), S. pectinacea (Hall), Orthis perversa (Hall), Streptorhynchus 

 pandora (Billings), Orthisina arctostriata and O. alternata (Hall), 

 Orthis inequalis and O. pravus (Hall). 



Among these there are noticed as distinct varieties a Streptorhynchus 

 pandora (Billings), which differs very little from S. woolworthana 

 (Hall sp.) of the Lower Helderberg group. 



b, Streptorhynchus arctostriata (Hall) ; 



c, Streptorhynchus perversa (Hall) ; 



d, Streptorhynchus pectinacea (Hall). 



These varieties of form and of surface-markings are illustrated on 

 Plates iv, ix and x of Vol. 4, Palaeontology of New- York. 



The study of the hinge-structure in some forms referred to the Genus 

 STROPHODONTA has Suggested some modifications in the limits of the genus, 

 and indicates in one direction an intimate relation with Streptorhyn- 

 chus. 



[Assem. No. 239.] 31 



