42 Fossils of the Lower Silurian RocJcs oj Canada, 



Fig. 3. 



Fig. 4. 



Fig. 5. 



Fig. 3. — Murchisonia gracilis. 

 " 4. — Pleurotomaria umbilicata. 

 " 5. — View of the under side of Pleurotomaria umbilicata. 



Murchisonia gracilis is a long slender spiral shell, generally about 

 the sizo and of the form represented in figure one. The number of the 

 whorls or turns made by the shell is from eight to ten. They are regularly 

 rounded, and crossed by fine striae, only to be seen in perfect specimens, and 

 ■which extend in a direction up and down the shell. From the outside of 

 the aperture a flattened band ascends in a spiral com-se to the apex, following 

 the centre of the whorls. Neither this, however, nor the striae are to be 

 seen, except upon specimens that are perfectly preserved. The fossil is 

 usually found in the condition of casts, that is where the shell having been 

 imbedded in the rock, has decayed, leaving an empty cavity or mould of its 

 shape. This having afterwards been filled up -vsith stone, gives a cast of 

 the shell, nistead of the petrified shell itself. Such specimens sometimes 

 only present the form of the interior of the fossil. In certain localities, 

 such as at the Chaudiere Falls, at the City of Ottawa ; at Paquette's Eapids, 

 in the Township of Westmeath, and at the third Chute of the Bonuechere 

 river, in the County of Renfrew, it is quite common, in the Black Eiver 

 and Trenton limestones. It is also found in the Hudson River group, but 

 we have never heard of its occurring in the Utica Slate. 



The genus Murchisonia was so named in honor of Sir Roderick Mur- 

 chison, at present the Director of the Geological Survey of Great Britain, 

 and the author of several magnificent works upon the Silurian rocks. It 

 was he who first worked out the Geology of those formations, and gave them 

 the name they now bear, and all the subsequent labours of geologists, in 

 this part of the series, are based upon the results of liis researches. The 

 specific name of this species is, in Latin, gracilis, " slender." The genus 

 contains a number of other very beautiful species, some of which shall 

 receive due notice in this journal. 



Pleurotomaria umbilicata. — This is another fossil usually found 

 in the condition of casts. The above figure 2 represents very correctly 

 a specimen from the Barrack Hill, at the City of Ottawa. In this specie 



