18 ECHINODERMATA. 



Type. Pentremites injlatus, Sow. Carboniferous Limestone. 



The genera are all extinct, and belong to the Palaeozoic rocks. One species appertains 

 to the Upper Silurian, six to the Devonian, and twenty-four are special to the Car- 

 boniferous rocks. 



Order vn. Cystoidea. — Body more or less spherical, supported on a jointed stem ; 

 the bursiform calyx is formed of close-fitting polygonal plates, varying in number in the 

 different genera, and investing the surface like a coat of mail, except above, where there 

 are three openings, one for the mouth, one for the vent, and one with a valve for the 

 reproductive organs ; the fourth aperture is below, and continuous with the canal in 

 the stem. Some have two or four arms, others are armless ; certain forms possess arti- 

 culated tentacula and curious comb-like appendages, or pectinated rhombs, in connection 

 with the plates. 



Type. Pseudocrinites quadrifasciatus, Pearce. Upper Silurian. 



This order is extinct. All the genera are found in the Silurian and Devonian rocks. 



Order vin. Crinoidea. — Body fusiform, distinct, formed of a calyx composed of a 

 definite number of plates, provided with five solid arms, independent of the visceral 

 cavity, and adapted for prehension ; mouth and vent distinct ; no retractile suckers ; 

 ovaries at the base of the arms opening into special apertures. Skeleton complicated, 

 calcareous, composed of thick plates closely articulated together ; their number 

 and arrangement are determinate in the different families, the multiples of five being 

 those which predominate ; the central plate of the body is supported on a long, 

 jointed column, that was firmly rooted to the sea-bottom. The mouth is central, and 

 prominent; the vent is situated at its side; the arms are mostly ramose and multi- 

 articulate, and when extended formed a net-like instrument of considerable dimensions. 

 The mouth is always placed upwards, and retained so by the column being jointed to the 

 central plate of the calyx. The normal station of the Crinoidea is the reverse of the 



ASTEROIDEA and ECHINOIDEA. 



Type. Pentacrinus Capnt-Medusa, Miller. From the seas of the Antilles. 

 Extinct families of Crinoids have lived in all seas from the Silurian upwards, and 

 only one or two representatives now exist. 



Prom the above analysis of the class Echinodermata, it appears that, as the Sipuncu- 

 loidea and Ilolothuroidea are not found in a fossil state, and the Blastoidea and Ci/stoidra 

 are special to the Palaeozoic rocks, our field of investigation in this Monograph is limited 

 to the Echinoidea, Asteroidea, Ophiuroidea, and Crinoidea, which we now propose 

 to consider seriatim, commencing with the Echinoidea. 



