474 Fossils of the Hamilton Group. 



slight depression extending along the posterior slope, giving a 

 contracted appearance to this part of the shell." 



The specific name appears to have been derived from the Latin, 

 hellus^ pretty. 



Fig. 7. Modiola concentrica. " Oblong-ovate, very inequilate- 

 ral ; surface covered with regular, equal concentric strite, which 

 become confluent towards the base ; hinge line curved ; anterior 

 side short, with a longitudinal impression directly below the 

 beaks." 



Generic name from the Latin, modiolus,^ a small measure or 

 drinking vessel. 



Fig. 8. Turho Uneatus. " Turbinate obtuse ; surface marked 

 by several sharp spiral lines, all which, except the central one, 

 are not visible on the cast; longitudinally striated, last whorl 

 of the shell rapidly expanding; aperture orbicular; umbilicus 

 moderate.'" 



Turbo Latin, a top, Uneatus, covered with lines. 



Fig. 9 & 10. Sjnrifer mucronatus. " Varying in form from 

 semicircular to triangular, with the hinge line greatly extended ; 

 surface marked by 24 to 30 rounded ribs, which are crossed by 

 crowded undulating lamellae, giving a squamous appearance to 

 the shell hinge ; area very narrow ; aperture small. 



Fig. 9 is the nearly semi-circular form; Fig. 10 shews the 

 hinge line more extended. " This is a very ornamental shell, 

 and its numerous varieties in form are very interesting. In the 

 soft calcareous shales of Western New York, it is shorter and 

 more rotund, while in the sandy shales and shaly sandstones of 

 the middle and eastern part of the State, it is greatly extended, 

 and its extremeties very acute. Occurs in all the localities of the 

 upper middle portion of the group." 



Spirifer, from the Lati n Sj ira, a spire, and fero, I bear ; mucro- 

 natus, sharp pointed. This shell occurs abundantly in the forma- 

 tion in western Canada. 



Fig. 11. Atrypa prisca. "Oblong, often nearly circular; 

 lower valve least convex, v/ith the beak scarcely prominent, and 

 pressed close to the beak of the upper valve ; upper valve very 

 convex ; front margin often advanced and a little depressed ; sur- 

 face radiated with numerous round strise, which bifurcate at irre- 

 gular intervals." " The specimens vary in size, and frequently are 

 flattened from compression, so that they do not present the rotund 

 form of the figure. 



