ALLORHYNCHUS 



509 



internal characters, with its peculiar external configuration, have been 

 observed among those studied, and only two of the species which have 

 been examined exhibit an entire absence of the median septum in the 

 brachial valve. 



Allorhynchus n. gek. 



allorhynchus heter0p8i8 (winchell) 



One of the species in which an entire absence of the brachial median 

 septum has been observed is RhynchoneUa heteropsis Winchell, but in 

 its external configuration it departs so widely from the typical form of 

 Pugnax that no one has ever suggested that it be considered as a member 

 of that genus. In figure 8 a series of cross-sections of this species is 



o. a Q. Q. Q Q 



Figure 8. — Cross-sections of the rostral Portion of Allorhynchus heteropsis {Winchell) 



This series of six cross-sections (X -V2) is from a specimen from the Kinderhook bed 



No. 4, at Burlington, Iowa 



shown. The species can not be legitimately considered as a member of 

 the genus Pugnax because of its external form, and on the other hand it 

 can not be included in any of the recognized rhynchonelloid genera which 

 it resembles externally, because of its internal features; it is therefore 

 taken as the type of a new genus which may be called Allorhynchus. The 

 species is marked by strong and angular plications which continue to the 

 beaks of both valves, and in the umbonal portion of the brachial valve 

 there is a distinct, median, longitudinal depression. 



ALLORHYNCHUS MACRA (HALL) 



Another species having the internal features of A. heteropsis, and lack- 

 ing the external form of Pugnax, is RhynchoneUa macra Hall, from the 

 Salem limestone. A series of cross-sections of this species are shown in 



Figure 9. — Cross-sections of the rostral Portion of Allorhynchus macra {Hall) 



This series of four cross-sections (X 2 14 ) is from a specimen from Salem limestone near 



Alton, Illinois 



figure 9. The species differs from A. heteropsis in its much smaller size, 

 its much more compressed form, its much shallower and more obscure 

 fold and sinus, and in the obsolescence of the plications toward the beak. 



