CONTRIBUTIONS TO PALAEONTOLOGY. 



45 



GENUS CENTRONELLA ( Billings,* 1859). 



" Generic characters. Shells having the general form of Tere- 

 BRATULA. Dorsal valve with a loop consisting of two delicate 

 riband-like lamellae, which extend about half the length. These 

 lamellae at first curve gently outwards, and then approach each 

 other gradually, until at their lower extremities they meet at 

 an acute angle ; then becoming united, they are reflected back- 

 wards towards the beak in what appears to be a thin flat ver- 

 tical plate. Near their origin, each bears upon the ventral side 

 a single triangular crural process. Name from the Greek, kentron^ 

 a spur. This genus is intermediate between Terebratula and 

 Waldheimia. In the former the loop is short, not exceeding 

 greatly one-third the length of the shell, and not reflected : in 

 the latter, it extends nearly to the front, and is reflected, but 

 the laminae are not united until after they are folded back." 



Fig. 13. Fig. 14. 



Fig. 13 [4]. luterior of the dorsal valve, showing the loop. 



Fig. 14 [5] . Longitudinal section, showing the position of the loop in the interior. 



In Centronella, as illustrated by Mr. Billings, 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 feature 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 known species of Gryptonella. 



Fig. 15. Ventral view. 



Cast of Centronella glans-fagea. 

 Fig. IG. Dorsal view. 



Fig. 17. Profile view. 



* Description and figures copied from tlie Canadian Naturalist and Geologist for 

 April 1859: the figures enlarged three diameters. 



