E. Billings on the Genus Centronella. 239 
matter of opinion, I have reason to believe that the internal 
organs of the fossil Brachiopoda are much more variable than is 
generally supposed. Some of these variations I shall mention 
presently. I shall first make some remarks on the punctate 
tra and YT. rectirostra.* And when afterwards he figured the 
muscular impressions of C. eximia, I concluded that all of the 
species belonged to Charionella. I do not now think the punc- 
tate structure of the shell a good guide in classification, as it is a 
character which pervades the Brachiopoda widely and irregu- 
larly, without regard to the affinities of the groups of species in 
which it occurs. ‘ 
The grounds upon which C. Julia is said to be separable gen- 
erically from C. glans-fagea are the following. 
. The species of Centronella heretofore described have the 
“ventral valve highly convex or subangular in the middle, with 
the dorsal valve flattened or concave in the middle, or with a 
median depression, and convex at the sides.” (Prof. Hall, p. 402.) 
In C. Julia both valves have a “regular lens-like convexity.” 
In answer to this I have only to state that in almost all genera 
of Brachiopoda, where the species are numerous, similar differ- 
ences in the form occur. Let us refer to Waldheimia the genus 
orm of C, Hecate. 
W. Waltoni, W. lagenalis, W. ornithocephala and W. Ceéltica are 
examples of elongate ovate forms like the typical species of 
Prof. Hall’s genus Cryptonella, In Terebrutula proper, numerous 
examples of similar and even greater differences might be cited. 
The hinge, socket, and dental plates are also liable to small 
Variations in structure in different species of the same genus, 
Thus, most species of Orthis have a well developed divaricator 
process in the dorsal valve. But in O. Hlectra and O. Tritonia 
there is not a vestige of this organ to be seer, the umbo being 
simply hollowed out into a triangular cavity, to the bottom of 
which the muscles for opening the valve were no doubt attached. 
O. porambonites, this process appears in a rudimentary form, 
* See the on M: Haskinsi is described as hav- 
ing the bechamel Far te shell faroca, with an exterior covering which ap- 
Pears to be punctate,” and on p. 85, M. Doris as having the “ shell-structure punctate.” 
