182 THE CANADIAN NATURALIST, [June 



deans, with numerous plates, sucli as Sphceronifes, Malocystltes, 

 Comaroajst'itcs, Amygdaloci/stites, and others, the shell is neither 

 annulated nor radiated, but composed of an indefinite number 

 of plates, reasing with the age of the individual, and arranged 

 without any well defined or constant order. It seems clear, there" 

 fore, that the test of the Cystidea belongs mostly to the periso- 

 matic system. 



In Peniremltes the three plates wdiich arc usually called the 

 basals, 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 , in their upper edges, for the re- 

 ception 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 believe it cer- 

 tainly does exist. It is said that the lower pieces consist of the 

 upper joint of the column, divided into three by vertical su- 

 tures. To me they appear to calycine plates. It is true that 

 they do not form the bottom of the visceral cavity, but this may 

 be due to the growth inward 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 by the basals, but after- 

 wards principally by the first radials. 



The forked plates are usually called " Radials,'^ but they cer- 

 tainly do 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. little con- 

 sideration 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 ambulacra originate and grow outward by the 

 addition of new plates to their distal extremities. There can be 

 little 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 down- 



