332 J. w. DAVIS ON THE genus plefeacanthfs, agass. 



come finer or altogether disappear near the point. On the posterior 

 :face there is a double row of obtusely pointed denticles two tenths 



Fig. 8. — PleuracantJius cylindricus, Davis. 



a. Section of spine, b. Portion of posterior surface, showing denticles. 

 c. Denticles, enlarged. 



of an inch apart, except near the apex (where the space between 

 them is much reduced), slightly curved towards the base. They ex- 

 tend from the point downwards 7^ inches, and increase somewhat 

 irregularly in size with the diameter of the spine or towards the 

 base. The denticles are firmly implanted, round near their base, 

 but contract and form a cone elongated transversely to the longitu- 

 dinal axis of the spine ; so that whilst the points are towards the 

 base, they are also turned decidedly away from the centre of the 

 spine (fig. 8 b). In the specimen figured by M. Agassiz*, a median 

 ridge is represented along the posterior face, between the two rows 

 of denticles, and continuing the whole length of the spine. In the 

 specimens I have examined this does not occur : there is sometimes 

 a level surface between the denticles ; but more frequently there is a 

 very decidedly hollow groove, which rarelj', however, extends far 

 beyond the termination of the denticles. 



A section of another spine is represented in fig. 8 a, in which, 

 the denticles are very widely separated. It is of the same species as 

 the specimen described above ; and aU the intermediate stages may 

 Tdo traced. In this instance the spine is of the same diameter as the 

 one represented in fig. 8 6, whilst the denticles are four tenths of an 

 inch apart, or double the distance. There is also in this example a 

 most decided groove, equidistant between the rows of denticles, and 

 quite one tenth of an inch across. 



Locality. T^ot uncommon in the Coal-measures. 



I am indebted to Mr. John Ward for placing at my disposal the 

 beautiful specimens from which the above descriptions have been 

 principally derived. They are from the ironstone shale at Fenton, 

 in Staffordshire. They are much larger and in better preservation 

 ihan any I have seen from the Yorkshire Coal-field. 



* Op. cit. 



