4 88 EXTINCT ORDERS OF ECHINODERMATA. 



plates ; it is supported upon a short jointed column like that of the 

 Crinoids (which is absent in Protocrmus), and in many genera arms 

 are developed which are comparable to those of Crinoids. There 

 are arnbulacral furrows, which are sometimes forked, and peculiar 

 organs termed " pectinated rhombs." 



The oldest known genera are Protoci/stites, of the Menevian beds, 

 and Macrocystella, which is also of Cambrian age. Other genera 

 occur in the primordial rocks of Bohemia and the Fichtelgebirge, 

 and PalcBOcystites is found in strata of the same 

 age in Canada. By far the larger number of Cys- 

 tidians is found in the Middle and and Upper 

 Cambrian rocks of the Continent. Echinosphcerites 

 occurs in the British Llandeilo rocks, but in Russia 

 the following genera also occur Sphceonites, Caryo- 

 cystites, CystoWastus, Asteroblastus, Blastoidocrinus, 

 and Mesites. Several genera occur in Sweden, such 

 as EucystiSj Holocystites, Glyptosphcerites, Glypto- 

 cystites, Gomphocystites, and Lepadocrinus. Bo- 

 hernia yields Trochocystites and RhombifercL, while 

 from the Canadian rocks many other genera are 

 described, such as Pleurocystites, Comarocystites, Amygdalocytes, 

 Malocystites, &c. The true Silurian rocks contain Ateleocystites and 

 Lepadocrinus. Both date from the Cambrian rocks, and the former 

 survives the Devonian. These, with Primocystites and Ecliino- 

 encrinus, are well-known Dudley fossils. In North America, Caryo- 

 crintts, Callocystites, Holocystites are also found of this age. The 

 Devonian strata yield Tiaracrinus and Echinocystites, which is also 

 found in the Silurian rocks of America. Agelacrimis, which com- 

 mences in the Cambrian, survives to the Carboniferous, in which 

 Lepadocrinus and"the embryonic crinoid Hypocrinus x are also found. 

 After the Carboniferous period Cystidians are unknown. 



Blastoidea. The Blastoidea is an extinct order limited to the 

 Primary rocks closely related to the Crinoidea and Cystoidea, which 

 it resembles in having a jointed column. Its test is made up of 

 polygonal plates ; there are large ambulacral areas but no arms. Mr. 

 Herbert Carpenter considers the type genus Pentremites to be absent 

 from the British rocks. The group first appears in the Silurian strata 

 of Tennessee, but is chiefly characteristic of the Devonian and Car- 

 boniferous periods. Pentremites, Nucleocrinus, and Eleutherocrinus 

 are characteristic of the Devonian rocks, but Granatocrinus and 

 Codonaster are common to the Devonian and Carboniferous. 



Echinoidea. The ancient sea urchins of the Primary rocks are 

 mostly distinguished by the shell being composed of more than 

 twenty rows of plates, with five or ten perforated plates in the apical 

 disc. These characters have been thought sufficient by Zittel to 

 separate the Palceechinoidea from the Euechinoidea, in which the 

 skeleton is formed of ten rows of ambulacral plates in five pairs, 

 alternating with five pairs of interambulacral plates, and the apical 

 1 See P. H. Carpenter, Q. J. G. S., vol. xxxviii. 



