BRACHIOPODA. 291 



In the Carboniferous limestone of Windsor, Nova Scotia, we find a very inter- 

 esting form, in the species described as Centronella Anna, Hartt,* where tlie long 

 and greatly recurved Cryptonella brachidium is retained with some accompany- 

 ing modifications in other features. It seems, beyond doubt, that Professor 

 Hartt misapprehended the structure of the brachidium in the shell. He has 

 represented it as somewhat similar to that of Centronella Julia, A. Winchell, the 

 descending branches uniting anteriorly to form a vertical median plate. By 

 good fortune there has been obtained an example of this rare shell, filled with 

 compact crystalline calcite, a most unusual condition of preservation in this 

 limestone ; and the demonstration of the brachidium from this specimen is very 

 complete. 



The external form of the shell is unusual, being plano-convex or naviculoid, 

 as in the typical species of the genus Centronella ; the brachial valve is 

 depressed-convex or nearly flat and the pedicle-valve medially ridged with 

 abrupt slopes at the sides. The dental lamellaj of the pedicle-valve are well 

 developed as in Cryptonella. In the brachial valve there is a short, tripartite 

 hinge-plate, supported by a median septum of considerable height in the um- 

 bonal region and extends for fully one-half the length of the valve, becoming 

 low anteriorly. 



The crura are very short and are continued almost immediately into the 

 long convergent crural apophyses The descending branches of the brachidium 

 extend for nearly the entire length of the shell, following the curvature of the 

 valve and approaching each other anteriorly, their extremities being again 

 directed outward. The ascending branches extend backward to points not far 

 in front of the crural apophyses, where they are united by a transverse band. 

 The outer margins of the descending lamelljB are fringed with rather long, 

 irregularly set spinules directed toward the commissure of the valves. There 

 are no spinules elsewhere on the brachidium. Although we are not inclined to 

 place a high value upon the presence of these spinules, they seem to be, in many 

 cases, a natural accompaniment of the brachidium in late palaeozoic species 

 (see observations on Athyris); but the entire combination of the centronellid 



* Hartt in Dawson'.s Acadian Geology, second ed., \>. 300, fig-. 90. 1868. 



