A distinct, usually high, cardinal area extends from the foramen to the 

 cardinal line. 



1 



Dorsal valve as in Acrotreta. 



The difference in the form of the 

 ventral valve distinguishes this genus 

 from Acrotreta and is accompanied by 

 difference of habit, etc. In Acrotreta 

 the vjsceral callus is concentrated 

 around the foraminal passage, and the 

 shell appears to have been of sedent- 

 ary habit, since the ventral valve in 

 many cases is found fossil in such an 

 attitude as to show that it stood in a 

 vertical position in the mud of the sea 

 bottom when the animal which inha- 

 bited it was living, the opening of the 

 valve being uppermost. No such uni- 

 formity of attitude characterizes the 

 dorsal valve. 



We find that the ventral valve in 

 Acrothyra assumes quite a different 

 attitude. It lies in very many cases 

 on its side, and usually with the open- 

 ing of the valve uppermost. Moreover 

 it is to be noted that on successive 

 layers these valves lie with the umbo 

 oriented in a fixed direction. From 

 this it may be inferred that they give 

 evidence herein of the action of a cur- 

 rent, flowing in a definite course and sweeping the valves in the direc- 

 tion towards which the current set. They may have swung in this direc- 

 tion by the pedicle while the animal was living ; or when swept away by 

 the flowing water, have presented the point of least resistance to the cur- 

 rent, as they sank to the bottom. In either case we must regard Acro- 

 thyra as living under different conditions from Acrotreta, which, as we 

 have remarked, apparently had the apex of the ventral valve buried in 

 the mud. It is in accordance with these conditions that we have in 

 Acrothyra a visceral callus developed along the median line of the ventral 

 valve, as is the case in Lingula and other allied genera ; and Lingula, 

 as is well known, had a long pedicle. 



Acrothyra proavia, mut. prima 

 1, Ventral valve 2, Mould of the same 

 3, Same in profile 4, Dorsal valve-5, 

 Mould of the same- 6, Same in profile. 

 All magnified S Upper Etchemin Shale, 

 C. Breton, N. S. 



