344 



ME. E. 9. COBEOLD OX THE 



fvol. lxxvi, 



the vascular sinuses of the ventral valve are usually, hut not 

 invariably, bent out widely so as to follow a curve that is sub- 

 parallel to the margin, instead of being straight ; the pedicle-tube 

 appears to be stouter and more triangular. 



From Dr. Kiser's figures the shell appears to have been thick, 

 and to have the retrousse margin that is so characteristic of the 

 Comley shells, but possibly this similarity may be due to its 

 preservation in shale. 



The variety comley ensis is nearer to the American form of 

 O. atlantica (that is, 0. rotunda fa), but not so near as the variety 

 transversa. 



Locality and horizon. — Comley, Lower Cambrian ; from the 

 Olenellus Limestone, horizon Ac.,, and from the Microdiscus- 

 bellimarcfinatus Limestone, horizon Ac.,. 



Oeolella atlantica, var. transversa nov. (PI. XXII, figs. 

 1-5; text-fig. 2.) 



This variety occurs at a higher horizon than comlet/ensis, and 

 always in association with a different group of trilobites. In 



Fig. 2. — Obolella atlantica, var. transversa nov. X 5. Diagrams 

 of internal casts of the ventral and dorsal valves, shoioincj 

 all marks observed in tlie several specimens. 



Ventral 



Dorsal 



point of size it is nearer to the American form, but differs in 

 having both valves more transverse and also in the details of the 

 internal marks. From the variety comleyensis it differs in general 

 shape and size ; in having a relatively thinner shell ; in the 

 external surface, which is more in accord with Walcott's descrip- 

 tion ; and in the internal features, which are proportionately 

 smaller. Though a larger shell, it appears to be built on more 

 delicate lines. 



Dimensions. — The lengths observed vary from 53 to 6"5 

 millimetres, the widths are from an eighth to a tenth greater, 

 the convexity averages about a fifth of the length. 



All the internal marks shown on several casts have been com- 

 bined in text-fig. 2. In order that the internal characters of the 



