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PROCEEMXCS OF THE XATIOXAL MUSEUM. 



VOL. XXXV. 



least synthetic, hence most characteristic phase. None of the species 

 belonging to the group suggests genetic relations to species not in- 

 cluded in the genus as here defined. 



GROUP OF B. KUCHIANA. 



In B. buchiana and its nearest allies and varieties the anterior and 

 median lobes are joined, as commonly happens in Beyrichia, but the 

 anterior lobe is thinner and the ventral connection proportionally 

 thicker than usual, the combination giving to the united lobes a strik- 

 ing similarity to the horse-shoe ridge in the B. ungula section of Bo£ 

 Int. Indeed, Beyrichia buchiana and the Cincinnatian Bollia regu- 

 laris (Emmons) and B. persulcata ( Ulrich) arc sufficiently alike in 

 general aspect to have induced so thorough a student of Ostracoda 

 as T. Rupert Jones to refer a partially covered specimen of the last 



to the Silurian Beyrichia* 

 12 13 The resemblance might be 



regarded as indicating ge- 

 netic affinity between the 

 two, the younger B. buchi- 

 ana being evolved through 

 the continued and finally 

 total obsolescence of the 

 anterior marginal ridge of 

 the Bollix. 



Though admitting the 

 possibility of such a deri- 

 vation, it has yet seemed to 

 the writers an improbable 

 relationship. T h e m a r - 

 ginal ridge is one of the 

 most stable characters of 

 Boll/a, and though its ventral part is often low and sometimes quite 

 obsolete, there is no evidence to show that the anterior part is even 

 ld-i entirely. The arrangement of the ridges in Bollia is bilateral 

 with respect to a median furrow, in Beyrichia with respect to a me- 

 dian lobe. In Boll in regularis the anterior marginal ridge is paired 

 with a less well-developed posterior ridge, the pair of median ridges 

 uniting below as usual. In testing the possible derivation of Bey- 

 richia buchiana from Bollia regularis, it should be remembered that 

 in the Beyrichia it is the anterior and median lobes that are united, 

 and thai if the suggested derivation were a fact, it must have been by 

 anterior shrinkage of the Bollia and final loss of the part bearing the- 

 anterior marginal ridge. Instead of this it seems certain that the 

 stronger of the terminal ridges on valves of B. regularis is the ante-: 

 rior one, proving that in this species at least the conditions are the 

 opposite of what they should be. 



Figs. 12-15. — 12-14. Left valve axd side and 

 edge views of another left valve of bollia 

 regularis (emmons), x 30. arnheim beds of 

 Richmond group, Waynesville, Ohio. 15. Left 

 valve of Beyrichia buchiana Jones, X 8 

 (after Jones 1. The figures are intended to 

 illustrate the fossible derivation of the 

 B. buchiana croup of Beyrichia from Bollia. 



