292 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXXV. 



The relations of B. tumida to the Clinton B. lata, suggested in 

 1894,° have been confirmed in the present investigation. Though 

 widely different in general aspect, the evolution of the latter from 

 the former is regarded as not unlikely. In the rapid and sometimes 

 extravagant mutation that is indicated, not only for the ostracoda 

 but also in other classes of animals, at and immediately following the 

 close of the Ordovician, it is readily conceivable that both the ante- 

 rior and the posterior lobes of B. tumida might have been greatly 

 reduced and thus to have brought about a temporary stage like B. lata. 



The Clinton speeies is 



24 



SJ 



25 



Figs. 23—26. — 23. Left valve op Drbpanella richard- 

 soni (Miller), < 10, introduced for comparison with 

 Beyriciiia tuberculata. Upper beds of the Rich- 

 mond GROUP, NEAR WILMINGTON, OHIO. -4. RlGHT 

 VALVE OF BEYRICHIA TUMIDA (ULRICH), < 10J SHOWING 

 ITS QERIVATION FROM DREPANELLA. TOP OF RICHMOND 

 GROUP, MORELAND, Kv. 25. RIGHT VALVE OF BEYRICHIA 

 LATA HALL, X 10, FOR COMPARISON WITH BEYRICHIA 

 TUMIDA AND DREPANELL1 RICHABDSONI. CLINTON" 



group. New Hartford, N. Y. 26. Right valve of 



TREPOSELLA LYONI (ULRICH), X 20, A DERIVATION OF 

 BEYRICHIA IN WHICH Till: POSTERIOR LOBE HAS BE- 

 COME obsolete. (After Clrich.) Onondaga lime- 

 stone, Falls of the Ohio. 



the loop formed by the ventral union of the 

 nodes, B. lata also suggests a Bollia. 



chiefly remarkable be- 

 cause of the slight de- 

 velopment of its pos- 

 terior lobe. In the 

 Devonian decadence 

 of typical Beyrichia a 

 similar obsolescence of 

 the posterior ridge is 

 noted in the modified 

 stage represented by 

 Treposella lyoni (Ul- 

 rich). At this time 

 the Beyrichiidse as- 

 sumed various atavis- 

 tic expressions, some 

 suggesting ( 'tenobol- 

 bina, others Bollia, 

 while a third may re" 

 call Tetradella. In 

 the partial obsoles- 

 cence of the posterior 

 lobe and the propor- 

 tionate distinct ne-- of 

 median and posterior 



GROUP OF B. SALTERIANA. 



The species of this group do not, as a rule, attain the average size 

 of those included in the />'. tuberculata, B. kloedeni, and B. buchiana 

 groups. They differ rather obviously, too. from these other groups 

 in tlir fullness of their lobes and the proportionate narrowness of the 

 furrow-. The departure from the more typical sections is toward 

 Klmdenia, in which the furrows are obscure or die out entirely in 



" Ulrich, Geol. and Nat. Hist. Surv. Minnesota. Final Rept., Ill, Ft. 2, p. G74. 



