135 



Fig. 14 (5). Longitudinal section, showing the position 

 of the loop in the interior. 



In Centronella, we have a simple loop, or the two limbs 

 becoming united at an acute angle at the point of greatest 

 anterior extension, whence they recurve in a thin vertical 

 plate which is not attached at either margin; approaching, 

 in some respects, to Waldheimia. 



This difference of the internal loop is accompanied, in 

 the cast of C. glans-fagea, the typical form of the genus, by 

 other differences which distinguish it from the casts of 

 Cryptonella. 



Fig. 15. Fig. 16. Fig. 17. 



Fig. 15. Ventral view of cast of Centronella glans-fagea. 

 Fig. 16. Dorsal view of cast of Centronella glans-fagea. 

 Fig. 17. Profile view of the same. 



In the cast of a ventral valve of C. glans-fagea, fig. 15, 

 we have the filling of a deep rostral cavity ; the dental 

 lamellae have been thick and strong, not extending as thin 

 plates into the cavity of the shell as shown in Cryptonella, 

 but having a thick blunt lower termination which leaves no 

 space, or scarcely an appreciable one, to be filled between 

 it and the shell. The spur, or filling of the rostral cavity 

 is striated ; at its base in the centre, on the body of the cast, 

 is a depression ; and on each side are fainter striated im- 

 pressions, indicating the points of muscular attachment. 



The interior of the ventral valve of Centronella impressa 1 



1 A very distinct species from C. hecate (Billings) of the Oriskany Sand- 

 stone, which differs mainly in size from Centronella (Rhynchonella ?) alveata 

 (Hall), Tenth Report on the State Cabinet, 1857. 



