no. 1040. REVISION OF BEYRICHI1D2E—ULRICR 1 V/> BA88LER. 295 



a thick, yet sharply defined, sickle-shaped ridge with two separate 

 rounded nodes above and a variable short ridge just within the ante- 

 rior edge. Except the interiorly concave marginal border, smaller 

 size, and proportionally narrow anterior end, the general aspect, es- 

 pecially in the matter of lobation, is highly suggestive of Drepanella. 

 Here again, however, the writers doubt the reality of the suggested 

 genetic relation. On the contrary, it is thought the true affinities of 

 the species lie with other middle Devonian species that could not be 

 suspected of alliance to Drepanella except in a very remote degree. 

 These species, namely, constitute a peculiar group, described and 

 mostly referred by Ulrich, as is now believed incorrectly, to Ctenobol- 

 hina. Conspicuous members of this group are C. informis, C. <tnfc- 

 spinosa, ('. Kpicirfosa, C. cavimarginata, and ('. insolens. These spe- 

 cies, it will be noted, vary greatly in general expression, and because 

 of their spinosity. probably indicate decadence of the Silurian tvpe 

 of Beyrichiidae, and rapid evolution toward the establishment of the 

 final, again comparatively long-lived type of the family. In the 

 transition, various atavistic stages are indicated, some recalling 

 Otenobolbina, some Bollia, and others, like B. kolmodini, more nearly 

 resembling Drepanella. Previously highly important and constant 

 features have become most unstable, but through all the vagaries the 

 steady evolution of the two rounded nodes which constitute the essen- 

 tial characteristic of the dominant and generically distinct later 

 Paleozoic Beyrichian type is manifest. These two nodes, which rep- 

 resent the median and anterior lobes* of typical Beyrichiae, are well de- 

 veloped in B. kolmodini, but the general expression of the valves in 

 this transitional stage in the development of the family is so at vari- 

 ance with that of the typical Silurian groups of Beyrichia that the 

 writers have decided to recognize it by erecting the new genus 

 II oil 'nut. 



Subgenus STETJSLOFFIA, new. 



Beyrichia (part) of Authors. 



Strepula (part) (if Authors. 



GROUP OF P.. LINNARSSONI. 



Bt yrichia antiqua, 11. acuta, B. simplex, 11. linnarssoni, B. signata, 

 B. beyrichioides, and probably B. erratica Krause, which is provision- 

 ally not included in the above list, constitute a peculiar group sug- 

 gesting Strepula in having thin, elevated ribs or crests running over 

 the surface of the valves. It is believed that these ribs served the 

 purpose of strengthening the valves and that they are developed in 

 genetically distinct groups of species. Depending primarily on the 

 lobation of the valves and on their form in deciding questions of 

 relationship, the group under consideration conforms in all essential 

 respects with typical Beyrichia. 



