ASTEROIDEA. 397 



ginal rows of plates, but is doubtfully separable from Palceaster ; 

 while the Ordovician and Silurian Palaster'ma (fig. 274, a) has the 

 disc still more extensively developed, and is further distinguished by 

 the fact that the plates of the adambulacral series which are placed 

 at the angles of the oral aperture are large and triangular. The 

 Silurian genus Palceodiscus, again, has a discoid form, the body being 

 pentagonal, without distinct arms, and the mouth being furnished 

 with five pairs of triangular oral plates. Lastly, the Silurian genus 

 Palceoco?na (fig. 274, c) is an aberrant form of this group, with slightly 

 prominent arms, laterally bordered by long spines, the intervals 

 between which are filled up by a netted membrane. 



In the Devonian rocks, various forms of the E?icrinasterice have 

 been detected, the chief genus in this formation being Aspidosoma, 

 in which the disc is of large size, with five small arms, and the 

 structural characters are in many respects peculiar. Thus, the arms 

 are covered superiorly with two or four rows of closely apposed 

 plates, which are continued over the disc, leaving a central star- 

 shaped space apparently covered only by a leathery skin ; while the 

 madreporite is stated to be placed interradially on the under side 

 near the mouth. The known species of this singular genus are 

 found in the Devonian rocks of Germany. Heliaiithaster, of the 

 Devonian rocks of Germany and Britain, is imperfectly known, and 

 may possibly be an Ophiuroid. It has a large disc, and from four- 

 teen to sixteen arms. 



The sub-class of the Euasteroidea is distinguished from the pre- 

 ceding by the fact that the pairs of ambulacral ossicles are placed 

 opposite to each other, and are directly united by their inner ends. 

 The members of this group appear in rocks as ancient as the De- 

 vonian, while all the Mesozoic, Kainozoic, and Recent Star-fishes 

 are referable to this series. 



The following classification of the Euasteroidea has recently been pro- 

 posed by Sladen, some of the less important families being omitted : — 



Order I. Phanerogoxia. — Marginal plates large and well devel- 

 oped ; papulae restricted to the dorsal surface. Ambulacral plates well 

 spaced ; tube-feet in two rows. 



Families. — Porcellanasteridae {Porcellanaster, Cte?iodisais), Astropect- 

 inidae {Astroftecten, Lm'dia), Pentagonasteridae (Astrogonium, Stellaster, 

 Goniodiscus), Antheneidae (Gom'aster), Pentacerotidas {Pentaceros). 



ORDER II. CRYPTOGONIA. — Marginal plates inconspicuous ; papulae 

 distributed over the whole body. Ambulacral plates crowded and narrow ; 

 tube-feet in two or four rows. 



Families. — Solasteridae (Solaster, Crossaster), Pterasteridas (Pteraster, 

 Hymenaster), Echinasteridae (Echi?iaster, Cribrelld)^ Asteriidae (Asterias), 

 Brisingidas (Brisinga). 



The forms of the Euasteroidea which have been detected in the 

 Devonian rocks belong mostly to extinct genera (Xenaster, Eoluidia, 



