156 



last one large, and, including the aperture, nearly one half the whole 

 length of the shell. The most projecting part of each whorl is just about 

 the middle ; above this mesial angulation the surface of the shell descends 

 with a gently concave or nearly flat slope directly to the suture ; below 

 the angulation the slope is similar to that above, but appears to be more 

 nearly flat. On the body whorl the angular edge is bevelled on its upper 

 surface by a narrow flat spiral band about one third of a line in width, so 

 situated that it slopes upwards and inwards. This whorl is more concave 

 on the upper side than the others. Below the margin there is a wide 

 shallow concave band about two lines in width, but so slightly depressed 

 as to be scarcely distinguishable. The body of the whorl below the mar- 

 gin is large and obtusely conical, and the aperture appears to have been 

 caniculated or efFuse at the lower angle. Surface unknown, but probably 

 finely striated. The aperture is not preserved in the specimen, but it is 

 evidently large, straight or gently concave on the upper side, and with the 

 outer lip moderately convex. 



Length about 18 hues ; width of body whorl about 10 lines. 



This group belongs to the Lower Silurian group of MurcMsonia, of 

 which M. perangidata of the Black River hmestone is the type. 



Locality and Formation. — Gait. Guelph formation ; Middle Silurian. 



Collector. — E. Billings. 



MUECHISONIA VlTELLIA. (N. Sp.) 

 Fig. 138. 



Description. — Spire conical, apical angle about 60°; whorls 5 or 6, 

 strongly ventricose, obtusely angular, and with a flat spiral band along the 

 middle ; above the band descending vrith a flat or very slightly concave 

 slope for three-fourths the distance to the suture ; in the remaining 

 one-fourth very gently convex ; suture deep ; below the band moderately 

 and imiformly convex. The upper whorls appear to become regularly 

 convex, both above and below the band. The band is gently concave or 

 nearly flat, and its plane is nearly parallel with the longitudinal axis of the 

 shell. Surface with fine striae curving backwards above, and forwards 

 below the band. Aperture unknown. 



Length of the only specimen collected about 18 lines. Width of the 

 last whorl 16 lines. 



Locality and Formation. — Gait. In the Guelph formation ; Middle 

 Silurian. 



Collector. — E. Billings. 



