BRACHIOPODA. 177 



Artificial Key to the Genera. 



A. Beak not marginal I. 



I. Shell cemented, calcareous, with no pedicle opening or notch (rarely free).. I. 

 I. Interior of each valve with a large, elevated muscular impression. 



XXV. Pholidops. 



1. Interior of each valve with three or four muscular impressions a. 



a. An S-shaped vascular sinus present XXIV. Craniella. 



a. No S-shaped vascular sinus present XXIII. Crania. 



I. Shell attached by pedicle, phosphatic ; pedicle opening or notch present 2. 



2. Pedicle opening a narrow groove, not extending to the margin of the shell, b. 



b. Pedicle groove very narrow, extending almost to the margin of the 



shell XIX. Orbiculoidea. 



b. Pedicle groove suboval, extending about half way from the beak to 



the margin of the valve II. 



II. Both valves convex, the pedicle the more so.. XXI. Schizotreta. 



11. Brachial valve very convex ; pedicle valve concave aa. 



aa. Pedicle opening surrounded by a depressed area. 



XX. Discinisca. 

 aa. Pedicle opening not surrounded by a depressed area. 



XXII. Rcemerella. 

 2. Pedicle opening round, abruptly truncating the apex c. 



c. Shell depressed-convex ; beak elevated XII. Acrolhele. 



c. Shell sub-conical 22. 



22. Pedicle valve with an apparent cardinal area bb. 



bb. Two wart-like protuberances on the umbo.. XI. Acrotreta. 



bb. No wart-like protuberances on the umbo... XIV. Iphidea. 

 22. Pedicle valve without apparent cardinal area. 



XIII. Linnarssonia. 



2. Pedicle opening a triangular notch d. 



d. Surface with prominent radiating pits XV. Trematis. 



d. Surface without radiating pits 33. 



33. Brachial valve extending beyond the pedicle valve on all sides, cc. 



cc. Surface with radiating strise XVI. Schizocrania. 



cc. Surface without radiating striae XVII. Lingulodiscina. 



33. Brachial valve not extending beyond the pedicle valve. 



XVIII. Schizobolus. 



B. Beak marginal II. 



II. Cardinal area absent or if present rarely straight and generally confined to 



pedicle valve ; hinged line curved or if straight very short 3 



3. Shell obolelloid — Small, thick-shelled, ovate in outline. Cardinal areas 



very short e, 



e. Shell calcareo-corneous II. Dicellomus. 



e. Shell calcareous 44. 



44. Umbo very conspicuous XXVI. Kutorgina 



44. Umbo not conspicuous I. Obolella. 



3. Shell trimerelloid — Large, thick-shelled. Cardinal area of pedicle valve 

 very high ; beak straight and very prominent. Beak of brachial valve 

 inconspicuous. Interior of each valve has a conspicuous platform ex- 

 tending from the beaks forward f. 



