no. 1646. REVISION OF BEYRIGHIIDM—VLRICR AND BASSLER. 297 



slightly above the rest of the swollen posterior lobe, and of which 

 it forms the greater part, while in some of the later Silurian species, 

 namely C. auricularis {BoWa <iuricularis Jones and Holl), and C. 

 minor (Bollia minor Krause), it is rendered even more prominent 

 by the almost total obsolescence of the posterior part of the ( 'tenobol- 

 bina bulb (the obsolete part corresponds to the posterior lobe of a 

 Beyrichia) . 



Having reached the stages of C. subcrassa and ('. fulcrata, a fur- 

 ther discrimination of the median lobe might result in a species like 

 C. impressa {Entorrds impress,/ Steusloff) and finally in one like 

 Beyrichia antiqua of the same author. In this last the median lobe 

 is at least as large as in the average Beyrichia, and the species differs 

 from the more usual types of this genus only in the less sharp defini- 



Fins. 39^4. — 39. Left valve of Ctenobolbina umbonata (Steusloff). 40. Rioht 

 valve of Ctenobolbina fulcrata (Ulbich). 41. Right valve of Ctenobolbina 

 impressa (Steusloff). 42. Left valve of Ctenobolbina subcbassa Ulrich. 

 43. Left valve of Beyrichia (Steusloffia) antiqua (Steusloff), X 20. 44. Left 

 valve of Beyrichia (Steusloffia) acuta (Krause). (Figs. 40 and 42 auk after 

 Ulrich, 39, 41, and 43 after Steusloff, and 44 after Krause. All X 20.) 



tion of the post-median furrow and in the slight elevation and gen- 

 eral lack of definition that pertains to both the anterior and posterior 

 lobes. 



It is probably significant that most of these ribbed or crested spe- 

 cies comprising the B. linnarssoni group are of Ordovician age, in 

 which rocks Ctenobolbina and Tetradella are the prevailing genera, 

 ami unribbed, true Beyrichias almost unknown. The group, there- 

 fore, may be viewed as an intermediate stage in the development of at 

 least one of the groups of Beyrichia from Ctenobolbina. 



If accurately figured, Krause's Strepula reticulata should perhaps 

 be referred to this group. On account of the proportionately elon- 

 gate form of its valves and the great width of its marginal frill, 

 the species would stand somewhat apart from the more typical repre- 

 sentatives of the group. Because of a similarly fringed and reticu- 

 lated Beyrichia in the Waldron shale of Indiana, it seems just pos- 



