No. 6.] J. W. DAWSON — SILURIAN FOSSILS. 343 



Pteronitella curta, Billings. 



More perfect specimens of this shell enable me to add to Mr. 

 Billings'* description, that the left valve is considerably more 

 convex than the right, and ornamented with concentric, crowded^ 

 raised lamellae. There are two muscular impressions, the ante- 

 rior small, oval and near the beak, the posterior large and round. 



3Iurchisonia, sp. 



In addition to M. Arisaigensis and 31. acicula, which are 

 common on the East Biver, there is a third species, much less 

 elongated than the former, and with a single revolving band in 

 the middle of the body whorl. The specimens are not very 

 perfect. 



Holopea^ sp. 



A species not distinguishable from U. sub-conica of Hall fronj 

 the L. Helderberg. 



Platyceras, sp. 



A small but beautifully perfect specimen of a conical and sotne- 

 what pyramidal Platyceras, with slight plications on one side. 

 It is not distinguishable from young shells of P. pyramidatum 

 of Hall from the Lower Helderberg; and is the only shell of 

 this type I have seen in Nova Scotia. 



• 



Orthoceras Pictoense, n. s. 



Transverse section oval, perhaps partly a result of pressure. 

 Chambers narrow, 8 in an inch in a specimen 1-5 inch in greatest 

 diameter. Shell scarcely tapering in five inches. Surface whea 

 perfectly preserved with delicate longitudinal striae. Siphuncle 

 not well seen but apparently inflated in the chambers. This is 

 seemingly a representative in our Upper Silurian of 0. hullatum 

 of England. 



Orthoceras (allied to 0. ibex). 



This species has long been known to me from Arisaig, and I 

 have specimens also from the East Biver, but not sufficient to 

 make absolutely certain its identity or difference. 



* PalEBOzoic Fossils of Canada. 



