no. 1646. REVISION OF BEYRICHIID&—ULRICH AND BASSLER. 298 



the swollen ventral half of the valves. While clearly intermediate 

 in character between typical Beyrichia and Klcedenia, it yet seems 

 unlikely that either was derived from the other through the saltt riana 

 group. On the contrary, a study of B. salteriana Jones, B. reuteri 

 Krause. B. plicata (Krause), and B. trilobata (Krause) seems to indi- 

 cate a closely knit line of development that diverged, like the B. lin- 

 narssoni group, from some early stage of Gtenobolbina. The resem- 

 blance exhibited by B. plicata to Gtenobolbina subcrassa, for instance, 

 is too close to be regarded as otherwise than genetic: and the agree- 

 ment between B. plicata and B. reuteri, and between the latter and 

 B. salteriana, is so intimate that a similar relationship seems 

 undeniable. 



Compared with the other sections of the genus, the salteriana group 

 agrees best with the B. tuberculata group in the isolation of its median 

 lobe. The dissection of the other two lobes occurring so generally in 

 that group, however, is not even suggested. 



28 29 



Figs. 27-.'!0. — 27. Right valve or Ctenobolbina subceassa Ulrich, X 20. (After 

 ulbich.) 28. illght valve of beyrichia plicata (krause), x 20. 20. left valve 

 of Beyrichia reuteri Krause, x 15. (Figs. 28 and 2!) are copied from Krause.) 

 30. Right valve of Beyrichia salteriana Jones, x 10. (After Recter.) The 

 figures illustrate the relation and probable derivation' of the beyrichia 

 salteriana group from a ctenobolbina like subcrassa. two intermediate stages 

 are shown in figs. 39 and 41. 



Beyrichia (/r<ntuh>s<i Hall, from the Waldron shale of Indiana, is 

 a good American example of this section of the genus. This species 

 is of exceptional interest because it is one of the few species of the 

 genus that occur in Silurian deposits of the Ohioan Province. 



GROUP OF B. CLAVATA. 



Of the foregoing groups, those of B. klcedeni, I'>. buchiana, and B. 

 tuberculata represent the fully established and most typical stages of 

 the genus. The group of B. salteriana evidently originated in some 

 species of Ctenobolbina and probably is the stock from which B. 

 hlcedeni was derived. It is also the only known group from which 

 the genus Klcedenia might have sprung. The small group of B. 

 clavata, which includes B. jonesii Boll and possibly />'. umbonata 

 Renter, likewise has a character suggesting an earlier genus, namely, 

 the posterior ridge curves forward along the ventral margin, and, 

 though attached to the slender isthmus connecting the anterior and 

 median lobes, is often distinguishable as far as the antero-ventral 

 angle where it merges into the great, pear-shaped anterior lobe. So 



