the Crinoidcca, Cystidea, and Blastoidea. 411 



the genera, Trochocystites and Eocystites^ liaving been dis- 

 covered in the primordial zone. No other Echinodcrms have 

 been found in rocks of so ancient a date. 



Next in order may be phiced those genera whose test is 

 composed of a definite number of plates, which have, to some 

 extent, a quinary arrangement. Thus Ghjptocystites^ EcJiino- 

 encrinifes, Ajjwcystites, and several others liave each four series 

 of calycine plates, of which there are four plates in the basal 

 and five in each of the other three series. The respiratory 

 areas or hydrospircs are reduced in number — ten to thirteen in 

 Glyptocysfites, and three in most of the other genera of the 

 group. Neither in the plates nor in the hydrospires is there 

 exhibited any tendency to a radiated arrangement. The most 

 ancient genus of this family is Glyptocystites, Avhich first ap- 

 pears in the Chazy Limestone, and seems to have become ex- 

 tinct in the Trenton. The other genera occur in various 

 horizons between the Chazy and the Devonian. 



In the genera 7/e?nzc(9s»iiVe5 and Caryocrinus the hydrospircs 

 in the upper part of the test converge toward but do not reach 

 the central point of the apex, thus forming the commence- 

 ment of that concentration and complete radiation which 

 is exhibited in the ambulacral canal-system of the higher 

 Echinoderms. In a former note {ante^ p. 259) it is pointed 

 out that Caryocrinus has thirty hydrospires — ten at the base 

 with their longer diagonals vertical, a zone of six round the 

 middle with their diagonals horizontal, and a third band of 

 fourteen around the upper part of the fossil. These latter 

 are represented in fig. 1 as if spread out on a plane sur- 

 face. On consulting this figure, it will be seen that the flat 

 tubes of the hydrospires, represented by the parallel lines, all 

 converge toward the central point from which the dotted lines 

 radiate. This point is the position of the mouth in the recent 

 Echinoderms ; but in Caryocrinus it is occupied by a large 

 solid imperforate plate. The hydrospires are arranged in five 

 groups. Commencing at mv and going round by 1, 2, &e., 

 there are four in the first group, one in the second, four in the 

 third, one in the fourth, and four in the fifth. These five 

 groups represent the five ambulacral canals of the recent 

 Echinoderms. In the specimen from which this diagram was 

 constructed there are the bases of fifteen free arms to be seen, 

 situated at the outer extremities of the dotted lines. At the 

 base of each arm there is a small pore (^?), which I believe to 



in which they occur : — a, an .arm or amhulacrum ; m v, uiouth and 

 vent comhinecl in a single aperture ; ?n v s, mouth, vent, and spiracle ; 

 //, ambulacral groove ; p, ovarian pore ; s, spiracle ; cj), convoluted 

 plate ; r, oesophageal ring. 



