112 



tideans, with numerous plates, such as Sphoenonites^ Malocystites, Coma- 

 rocystites, Amygdalocystites, and others, the shell is neither annulated 

 nor radiated, but composed of an indefinite number of plates, increasing 

 with the age of the individual, and arranged without any well defined or 

 constant order. It seems clear, therefore, that the test of the Cystidea 

 belongs mostly to the perisomatic system. 



In Pentreniltes the three plates which are usually called the basalSy 

 consist each of two pieces, one placed above the other, and, in general, 

 closely anchylosed together. The lower pieces have each a re-entering 

 angle, m their upper edges, for the reception of the upper pieces which 

 stand upon them. This structure was first pointed out by Mr. Lyon 

 (Geol. Ky., vol. iii, p. 468), and is not generally admitted, although I 

 beheve it certainly does exist. It is said that the lower pieces consist of 

 the upper joint of the column, divided into three by vertical sutures. To 

 me they appear to be calycine plates. It is true that they do not form 

 the bottom of the visceral cavity, but this may be due to the growth in- 

 ward of the lower edges of those of the upper series. Something like this 

 occurs in Antedon, where, at first, the bottom of the cup is formed hj 

 the basals, but afterwards principally by the first radials. 



The forked plates are usually called " Picidials " but they certainly da 

 not belong to the radial system. If they did, they would represent the 

 first radials of the Crinoidea, and therefore they should support the bases 

 of the ambulacra. A httle consideration will, however, enable any one to- 

 perceive that in Pentremites the bases of the ambulacra, are situated in 

 the apex of the fossil, and do not come in contact with the forked plates. 

 The apex of Pentremites is identical with the actinal centre of Sea-urchins 

 and Star-fishes, in which the mouth is situated. It is here that the am- 

 bulacra originate and grow outward by the addition of new plates to their 

 distal extremities. There can be Httle doubt that such was the mode of 

 growth of the ambulacra of the Pentremites- The smaller extremity^ 

 therefore, of their ambulacra, which is received into the forked plate, is 

 not the base, but corresponds with the apex of the ambulacrum of a Sea- 

 urchin or of a Star-fish. It also represents the tip of the arm of a Crinoid. 

 If the forked plate is radial, then the arrangement of the ambulacrum 

 must be the same as that which would be exhibited in a Crinoid, with the 

 upper end of the arm downward, and resting on the first radial, whilst the 

 lower end would be upward, the tip being formed of the second radial. 

 From this it follows that the forked plates do not belong to the radial, but 

 to the perisomatic system. 



The five deltoid plates alternate with the forked plates, and are also 

 perisomatic. 



