Fossils of the Lower Silurian RocTcs of Canada. 43 



there are three elevated ridges or keels which follow the spiral curvings of 

 the whorls, and produce the angular form seen in the figure. The first of 

 these is situated at the bottom of the whorl, and the side of the shell rises 

 perpendicularly from it to the second placed upon the upper and outer mar- 

 gin — thence there is a curve still upwards, but inwards to the third keel. — 

 Above the first whorl, only two of the keels are visible, the other being 

 buried in the spiral suture between the whorls. The number of whorls or 

 volutions is about four, but some specimens shew more than these. This 

 fossil is seldom found in a perfect state, but even the iragments are easily 

 recognized after a little practice. Figure 3 shews the under-side of a speci- 

 men, with the umbilicus or cavity in the centre, around which the whorls 

 are twisted. 



The perfect shells of this genus have a notch more or less deep in the 

 outer margin of the mouth or aperture, and hence the name Pleurotomaria, 

 from pleura, side ; and toma, a notch. The specific name of this species 

 was given in allusion to the deep umbilicus. It occurs very commonly in 

 the Black Eiver and Trenton Limestones. 



/Mk 



% 





77/1 I 

 %#///// ;iiiiU\\\\\ ; 



mil M^^m^ 



^'-'^lliiiiiiiiil'iLi 



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Fig 6. Cyrtolites ornatus. Fig 7. AmbonycMa radiata. 



The first of the above named fossils, like the two preceding, is the shell 

 of a gasteroped, the class of which the existing land snails are well known 

 examples. It is a thin symmatrical shell, and is in its form simply an angu- 

 lar tube, partly coiled up at its smaller extremity. There is no spire, as in 

 the snails, but each side of the coil is equally depressed. The volutions are 

 two or three, — there is a sharp keel on the back and a deep groove on the 

 ventral or inside, next the whorls. The sides are also augulated, and the 

 aperture of a quadrangular shape. The dorsal slopes are marked, says Pro- 

 fessor Hall, " by strong transverse ridges, which extend to the angle at tiie 

 sides of the volution ; the surface is marked by fine transverse strise, the 

 spaces between which are crossed by fine curving ones, giving the surface a 

 cancellated or pitted appearance." 



'• This fossil usually occurs in the form of casts of the interior, which 

 preserve the form of the shell, the dorsal carina, and the transverse ridges, 

 but not the finer sculpture of the surface." In the perfect specimens, the 



