336 NORTH AMERICAN INDEX FOSSILS. 



44. Surface smooth cc. 



cc. Marginal ridge faint or absent ff. 



ff. Sulcus not bounded by prominent nodes *. 



^. Median depression broad, undefined IX. Primiiiella. 



"*". Median depression a well defined sulcus i^^. 



\''. Sulcus narrow, not extending below the middle. 



X. Primitia. 

 \^' . Sulcus broad, extending nearly to ventral margin. 



XV. Dilobella. 

 tf. Sulcus with a prominent rounded node on each side. 



XIII. Ulrichia. 

 t|. Sulcus surrounded by swollen node-like or ridge-like surface 



of shell XXXV. Beyrichidla. 



cc. Marginal ridge pronounced fft- 



Iff. Sulcus with one or two strong spines on each side, marginal 



ridge continuous XIX. Dicranella. 



ft f. Sulcus with prominent elongated rounded nodes only, mar- 

 ginal ridge not complete XX. Drepanella. 



44. Surface pitted XIV. Halliella. 



3. With two sulci, producing three unequal lobes d. 



d. Sulci vertical, mostly straight 55. 



55. Anterior node sometimes compound; margin with flat overhanging 



flange XXIX. Beyrichia. 



55. Anterior and posterior nodes confluent with shell ; sulci short ; mar- 

 ginal rim faint or absent dd. 



dd. Valves subequal XXX. Kloedenia. 



dd. Valves unequal, the right overlapping the left. 



XXXIV. Klcedenella. 



d. Sulci curved and oblique, anterior one generally much weaker or even 

 obsolete XXVIII. Ctenobolbina. 



3. With three sulci, the intermediate nodes often narrow and ridge-like e. 



e. A marginal or submarginal rim formed by ventral union of outer nodes..66. 



66. Inner ridges not swollen at ends, and uniting with marginal rim...^<f. 

 ee. Ridges thin, often dividing, no spines present. 



XXVII. Tetradella. 

 ee. Ridges thick, posterior one with horn-like or mushroom-like 



process XXVI. Ceratopsis. 



66. Inner ridges thick, uniting ventrally into a horse-shoe-shaped ridge. 



ff. Horse-shoe ridge large, often swollen at the ends... XXV. Bollia. 

 ff. Horse-shoe ridge minute, dorsally situated XXIV. Placentula. 



e. No distinct marginal rim, all the lobes irregular and confluent 77. 



77. Right valve the larger XXXIV. Klcedenella.. 



77. Left valve the larger XXXVI. Jonesina. 



I. Shell with ridges, nodes or spines, but with the sulci not definitely developed... 4. 



4. No marginal flange or rim, but supramarginal ridges may occur ./. 



/. Surface with two narrow ridges, converging ventrally.. I. Hipponicharion. 



f. Surface with curved ridge, generally parallel to basal margin 88 



88. Ridge narrow, elevated .gg- 



gg. Ridges U-shaped or crosier-shaped, no further nodes. 



XVIII. Jonesella. 



