no. 1646. REYISIOA OF BEYRICHIIDM—ULRICB AND BASSLER. 309 



Genus CTENOBOLBINA Ulrich. 



CtenobolMna Ulrich, Jour. Cincinnati Soc. Nat. Hist.. XIII, 1890, p. 108. 

 ct( nobolbina Miller, North Amer. Geol. ami Pal.. First App., L892, p. TOO. 

 Bollia (part) Krause, Zeits. d. d. geol. Gesell., XLIV. L892, p. 392. 

 Entomis (part) Steusloff, Zeits. d. d. geol. Gesell.. XLVI, 1894, p. 780. 

 CtenobolMna Ulbich, Geol. and Nat. Hist. Surv. Minnesota, Final Kept., Ill, 



Pt. i.'. 1894, p. <;:::. 

 CtenobolMna (Jrabau, Bull. Buffalo Soc. Nat. Sri.. VI. 1899, p. 300. 

 CtenobolMna Ulbich, Jour. Cincinnati Soc. Nat. Hist.. XIX. 1900, p. 180. 

 BeyHchia (part) of Authors. 



Carapace small, usually loss than -2 mm. in length, subquadrate or 

 subovate in outline, the hinge line long and straight; posterior two- 

 fifths more or less decidedly bulbous or subglobular in the typical 

 section of the genus, but in the C. subcrassa section the corresponding- 

 parts of the carapace are smaller and usually of lesser thickness than 

 certain portions in front of it. In the latter section a small node (the 

 homologue of the median lobe of Beyrichia ) is sometimes distinguish- 

 able on the inner slope of the posterior lobe. One deep, long, narrow, 

 generally curved and more or less oblique sulcus extends from the 

 middle of the dorsal (^]^e toward the post-ventral angle, occasionally 

 reaching the border. Area in front of median sulcus either simply 

 convex or divided by a shallower furrow usually paralleling the main 

 sulcus. Valves equal, the free edges thick, the contact margins 

 generally concealed, partly or wholly, in a lateral view, by a vari- 

 ously modified overhanging border. Surface granulose, smooth, or 

 punctate. 



Genotype, — CtenobolMna ciliata {BeyHchia ciliata Emmons). 



Since 181)0, when this genus was first described. Ulrich has on two 

 occasions (both cited above) added to the list of specie- originally 

 referred to the genus. Recent studies of the family have convinced 

 the writers that a good part of these later additions represents, as 

 indicated on page 295, atavistic Devonian stages in the development 

 and decadence of the predominating Silurian phase of the family. 

 Admitting this as probably true, it is thought desirable and of 

 distinct advantage in classification to remove these species from 

 Ctt nobolbina and to refer them, together with a few species hitherto 

 placed with Beyrichia and Bollia, to a new genus for which the name 

 Hollina is proposed on a following page. 



Even after the elimination of this peculiar Devonian group, the 

 remaining species fall into two easily distinguishable subgenera or 

 sections of the genus. The first of these two groups includes ('. 

 ciliata and its immediate Ordovician allies — all of them with a 

 granulose surface ornament — one early Trenton, one Silurian species 

 with finely reticulate surface, one Helderbergian, and one middle 

 Devonian papillose species. This section is characterized by the 



