no. 1646. REVISION OF BEYRICHIIDJE—ULRICH AND BAS8LER. 3^3 



cussed on pages 289 and 294. It is a remarkable fact that these two 

 alliances are more obvious and apparently more intimate than are 

 those between Drepanella and such nearly equally old genera as 

 Ctenobolbina and Tetradella. There is so little evidence of transition 

 between them that derivation of either of the latter from the first, 

 or of the first from either of the latter, seems out of the question. 

 Though it is highly probable that all three were derived out of the 

 same stock — presumably Primitia — it seems certain that the de- 

 partures from that primitive line were entirely independent and in 

 all cases rapid. Indeed, Primitia itself, which contains the oldest 

 of the distinctly furrowed Ostracoda, does not, geologically speaking, 

 greatly antedate Drepanella. The Cambrian seems to contain no 

 Ostracoda at all,° the oldest known representatives of the class being 

 Leperditiidse, found in rocks of Beekmantown age. The Primitiidso 

 and Beyrichiidse did not appear till post-Beekmantown. 



The species described in 1894 by Ulrich as Drepanella bilater- 

 al is '' is so peculiar that it seems unwise to continue listing it as a 

 species of this genus. Though exhibiting a general resemblance to D. 

 crassinoda, it seems on closer comparison that the surface lobation 

 is really very different. In the first place, the marginal ridge is 

 developed only along the ventral border, terminating abruptly on 

 both sides, when it begins to turn up on the ends. Next the nodes 

 bove the ridge do not correlate satisfactorily with those of airy of 

 the other species. There is a bilateral S3unmetry in their form and 

 arrangement with respect to the small mid-dorsal node that can not 

 be duplicated in typical Drepanella, nor readily explained. The 

 explanations occurring to the writers entail departures from that 

 generic type of such importance that the removal of the species from 

 Drepanella seems imperative. If the posterior node is assumed to 

 be in part made up of the post-dorsal portion of the marginal ridge, 

 then its inner part must represent the lower two-thirds of the trino- 

 date post-median ridge of D. crassinoda and the mid-dorsal node the 

 upper third of that ridge. According to another interpretation the 

 mid-dorsal node of D. bilateralis would correspond to the dorsal 

 part of the antero-median node of D. crassinoda and the main but 

 antero-median node, together with the crescentic ridge beneath it and 

 the small antero-dorsal node in the latter. In either case it would 

 mean that the mid-dorsal node occupies a different position from the 

 corresponding part of typical Drepanella; also modification of the 

 posterior lobes scarcely compatible with a strict conception of 



°A comprehensive study of the supposed Cambrian Ostracoda recently com- 

 pleted has led to the conviction that these are Phylloearida and Diol Ostracoda. 



b Geol. and Nat. Hist. Surv. Minnesota, Final Kept.. Ill, Pt. 2, ispi. p. 671, 

 pi. xlvi, figs. 35-38. 



