312 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxv. 



dorsal regions with two to seven nodes, the larger numbers resulting 

 through dissection of the primary two. When only two, they may 

 form a loop by union of their ventral parts. Surface smooth or 

 coarsely reticulated. 



Genotype. — Drepanella crassinoda Ulrich. Other species referred 

 here, all of Ordovician age,' are: D. ampla Ulrich, D. bigeneris Ul- 

 rich, D. crassinoda nitida Ulrich, D. elongata Ulrich, D. man-it 

 Ulrich, D. richardsoni {Beyrichia richardsoni Miller) and D. rich- 

 ardsoni canadensis Ulrich. 



This apparently wholly American genus is remarkable for the ex- 

 treme variability of the nodes within the central area of the valves. 

 The binodose D. ampla probably represents the most simple type. 

 From this we pass to D. elongata, with its ventrally fuller valves and 

 Klcedenia-like reduction and disposition of the nodes. There is a de- 

 jiression or sulcus between the nodes in this species. A similar de- 

 pression of the surface outside of the nodes, without a reduction in 

 altitude of the nodes and the lower boundary of the median sulcus, 

 would result in a form essentially like D. bigeneris, which is strik- 

 ingly like a Bollia. In D. macra, D. crassinoda, and D. nitida the 

 nodes range in number from three in the first to seven in the last. A 

 comparison of the nodes of these three species established beyond 

 question that the larger numbers are produced by dissection. Indeed, 

 the seven nodes of D. nitida are all indicated by corresponding wholly 

 or partially separated nodes in D. crassinoda, and the corresponding 

 parts are no less easily recognized in D. macro,. 



The only constant features of Drepanella are the sickle-shaped 

 submarginal ridge, and, within reasonable bounds, the size of the 

 carapace. In other respects the species are sometimes highly sug- 

 gestive of in part probably very distinct contemporary and later 

 genera. Thus, as stated on page 303, D. elongata might be classed as 

 a Klcedenia if it had not the characteristic, submarginal ridge, while 

 it is really difficult to point out sufficient reasons for excluding D. 

 bigeneris from Bollia. But Drepanella is an old genus — probably 

 the oldest of the true rigid Beyrichiida^ — having been already well 

 established in the Stones River epoch. These diverse resemblances 

 may, therefore, be explained as synthetic vacillations of an ancient 

 type prior to the fixation of generic characters marking later devel- 

 opmental stages within the family. The sickle-shaped ridge, how- 

 ever, was a fixed character and doubtless left its imprint in the his- 

 tory of the family. It is, therefore, not surprising that in the de- 

 cadence of the main Silurian genus Beyrichia this ridge is again 

 occasionally recognized. It is well shown, for instance, in the pe- 

 culiar Devonian descendant of Beyrichia, Hollina kolmodini (Jones). 



The probable relations of Drepanella to Beyrichia, especially to 

 the tuberculata and the elavata sections, have been sufficiently dis- 



