296 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxv. 



Beside the presumably dominating alliance of the B. linnarssoni 

 group to Beyrichia s. s., and the suggested relation to Strepula, other 

 in part apparently true alliances are indicated, in some by the ar- 

 rangement of the superficial ribbing, in others by the form and dis- 

 position of the lobes. Thus the tetrameroid arrangement of the ribs 

 in B. erratica and B. signata recalls Tetradella. The same may be 

 said of certain more typical and possibly true strepulse like S. lineata 

 Krause and the two varieties described by Steusloff as granulosa and 

 separata. The more simple B. antiqua Steusloff and B. acuta Krause 

 are like certain species of Ctenobolbina. In the opinion of the 

 writers, (his resemblance is of real genetic significance, the indicated 

 relationship and probable derivation of at least some Beyrichia' from 

 Ctenobolbina seeming fairly easy to establish. 



In tracing out this relationship we begin, not with the genotype, 

 ('. ciliata, and the four or five closely allied species found in the 

 Cincinnati rocks, but with the older Stones Eiver and Mohawkian 



35 



Pigs. 34-38. — .".4. Left valve of Beyrichia (Steusloffia) linnarssoni (Krause), X 20. 

 (After Krause.) 35. Right valve of Strepula concentkica Junes and IIoll, 

 X 15. (After Jones and Hole.) 30. Left valve of Strepula irregularis Jones 

 and IIoll, x 15. (After Jones and IIoll.) :'.7. Left valve of Beyrichia (Tetra- 

 della?) erratica Krause. (After Krause.) 38. Left valve of Strepula? lineata 

 granulosa Steusloff, x 20. (After Steusloff.) Shows similar development 

 of superficial linear crests in Steusloffia and Stkepula. In true Strepi i.a 

 tin: bexrichian lobes are not clearly determinable. 



forms. In the ciliata section of Ctenobolbina the median lobe is un- 

 distinguishably merged in the larger posterior bulb which charac- 

 terizes this section. It began in species like the early Trenton C. 

 obliqua Ulrich and the foreign Ordovician C. oblonga (Entomis 

 oblonga Steusloff), in which this median lobe is merely indicated by 

 the abruptness of the inner slope of the main lobe: and these species 

 seem to have been derived from the previously established C. sub- 

 crassa section. 



In the subcrassa section the median lobe is generally distinguish- 

 able, appearing as a small or larger node or ridge situated imme- 

 diately behind the main, median sulcus. Usually the posterior side 

 of the lobe is not sharply denned from the more or less swollen sur- 

 face behind it. Sometimes, as in C. umbonata (Entomis umbonata 

 Steusloff) and ('. subcrassa Ulrich, it forms a small, rounded node 

 on the inner slope of the main posterior bulb. In others (as, for 

 instance. C. crassa and ('. fulcrata Ulrich) it makes a low ridge rising 



