48 CANADIAN FOSSILS. 



folded round and brought up into their proper position, the mouth 

 would lie partly in the suture between two of the plates of the 

 second series, which is its position in many of the ordinary Cystideae 

 which have the definite number and arrangement of the plates. 



The ambulacral orifice probably lies in the apex in the bottom of 

 the groove between the two arms, and must be exceedingly minute, 

 as I have been unable to detect it in several specimens that I have 

 disarticulated for that purpose. 



The anal? arperture is also very small, and situated on the anterior 

 side, close to the apex. It consists of a small rugged notch in the 

 edges of two of the small plates, below the arms. 



The arms are articulated in two series, and the ambulacral groove 

 is continuous from one to the other across the space between their 

 bases. On each side of the groove is a row of small marginal plates, 

 two or three to each joint of the arm. 



The ijecdnated rhombs are three in number ; one of them is placed 

 half on the right dorsal basal plate and half on the large hexagonal 

 plate of the second series ; the other two are also situated on the 

 dorsal side, one upon the left pair and the other on the right pair of 

 plates of the third series. Of these two rhombs the left one is the 

 larger in all the specimens I have seen. 



This genus was first discovered in the Trenton limestone at the 

 city of Ottawa, and afterwards at Montreal in the same rock. In 

 1856 Mr. Richardson found one species in the Hudson River group 

 at Anticosti. In the Museum of Practical Geology, Jermyn Street, 

 London, are specimens of a species, P. Rvgeri (Salter), collected in 

 the Caradoc formation of Wales ; and M. Barrande informs me that 

 a species, which appears to be referable to the genus, occurs in his 

 fitage D, in Bohemia, the equivalent of the Landeilo and Caradoc 

 of Britain, and of the Trenton and Hudson River groups of America. 



The species are so closely allied, that there is much difficulty in 

 defining them ; yet I am satisfied that there are several which are 

 distinct. It is very improbable that a genus of Cystideans, ranging 

 from the Chazy limestone to the Hudson River group, and spreading 

 over so vast a geographical area as that which lies between Canada 

 West and Bohemia, should have but one species. Still it is not 

 easy to find many good specific characters to separate those in the 

 collection at Montreal. The following appear to me to be distinct; 

 but when the genus becomes better known, it may be necessary to 

 make some other disposition of them : — 



