70 CANADIAN FOSSILS. 



seen, that all the striae are covered by a thin exteiiial layer, which 

 closed them completely over, so that there were no pore-mouths on 

 the exterior. It therefore follows, that what appears to be ordinary 

 striae are in fact the channels of a peculiar set of pores which run 

 beneath the surfaces of the plate, and communicate with the interior 

 through orifices in the suture. Hence in a perfect plate, the margin 

 on the inside is deeply notched, each notch being one-half of a pore- 

 mouth. On the outside it is not notched. The pores do not open 

 any where upon the surface of the plate, neither do they communicate 

 with each other. When the plates are perfect, the surface is still 

 striated ; but the striae correspond to, and are situated over, the 

 partitions between the channels of the pores which constitute the 

 striae of the worn plates. The number of pores in the sutures depends 

 upon the size of the plates. In a plate, with sides two lines in 

 length, there are nine pores. 



Geological Position and Locality. — Detached plates, common in the 

 Chazy limestone at Caughnawaga and on the Island of Montreal. 

 They also occur in the same formation near Hawkesbury, and at 

 Chazy, in the State of New York. 



Collectors. — Sir W. E. Logan, J. Richardson, E. Billings, R. Bell, 

 and J. McMuUen. 



XVII. Pal^ocystites Dawsoni, Billings. 



Descrij^tion. — Of this species I have collected several fragments of 

 the body, two of which are especially interesting, as they exhibit 

 the exterior of some of the plates, and thus afford the means of 

 shewing that they are generically related to the last species. The 

 body is elongated pyriform, or rather fusiform, the lower part being 

 the smaller extremity. The apex appears to be rounded. The 

 plates are ornamented with strongly elevated rounded ridges, the 

 number of which corresponds to the number of the sides ; those with 

 five sides having five ridges, and those with six or seven sides, six or 

 seven ridges. Each ridge is continued from the centre of one plate 

 straight across the suture to the centre of the next. There are no 

 pore-mouths on the outside ; but on the inside there is one for each 

 ridge, situated on the suture, exactly as in the last species. The 

 pores in this species do not penetrate so far towards the centre as in 

 the last. 



The largest specimen collected, is a fragment of the lower half, 

 and indicates that the length of the body was about one inch, its 



