26 GEOLOGICAL MEMOIRS. 



In these lateral plates, in which the angle pointing towards the 

 vertex is truncated, each of the two angles thus formed seems to 

 represent those pointing vertically upwards, and is marked by a 

 corresponding double row of pores proceeding from the centre of 

 the plate. 



The basal or pelvic plates are decorated with pores after a manner 

 precisely similar to that just described. Here also a double row 

 proceeds towards the vertical angle of the two smaller four-sided 

 plates, but in the case of the larger six-sided ones, two double rows 

 proceed, diverging from the base towards the two angles pointing 

 upwards, thus showing distinctly the compound nature of these 

 plates, which evidently consist of a pair, exactly similar to the smaller 

 ones, but fastened together. In the basal plates many of the pores 

 towards the stem are covered with vesicles, but not so invariably as 

 the plates under the arms. 



Such is the decoration of the external surface of the Caryocrinus 

 in the young and mature state, but with age it appears to exhibit a 

 remarkable change. Along each row of pores there rise small ob- 

 long vesicles resembling the pores, but never piercing through and 

 reaching the inner surface of the plate. These vesicles gradually 

 lengthen, and at last unite into an elevated ridge, occurring between 

 the rows of pores when they are double, but, when the row is single, 

 under it, on the upper half of the plate, and above it, on the lower 

 half — another very singular instance of symmetry of arrangement in 

 this part. These ridges seem to have gradually risen in height while 

 the rows of pores remained buried by their side, so that the whole 

 plate at length puts on the appearance of a large six-rayed star, the 

 surface being separated by that number of ridges proceeding from 

 the centre ; and this radiated appearance is exhibited very beauti- 

 fully and distinctly even in the minute plates on the summit. The 

 rays always proceed from the centre towards the angle of the plate, 

 and never terminate upon the side ; and this is an important point, 

 because these radiating ridges differ essentially from the plications, 

 often very strongly marked, which pass on from one plate to another, 

 and frequently give to the surface a new polyhedral form, which a 

 hasty examination attributes to the form of the plates, although, in 

 fact, it is merely a modification of the external surface. 



But neither is this latter modification absent in the Caryocrinites, 

 which seem as it were to combine within themselves all the pecu- 

 liarities of form occasionally met with, and so singularly charac- 

 terising the various genera of Crinoidea, The striae of growth on 

 the surface of the Caryocrinus are distributed in a very distinct 

 manner as little granulations placed in lines intersecting one another 

 at right angles. However delicate these lines may be, their mutual 

 parallelism may always be traced, and so may also their position per- 

 pendicular to the edges of the surface on which they exist. They 

 appear to pass without interruption from the middle of one plate to the 

 middle of the next adjacent one. 



It is in this slight, and, for that very reason, instructive com- 

 mencement of an appearance — exhibited in so remarkable a manner 



