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 the 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 lamellae 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 uin- 
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 lamellae 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., p. 300, fig. 99. 1868. 
