ASTEROIDEA. 249 



pyramidal, sharp-edged ; skeleton of imbricate plates ; dorsal wart 

 single, rarely double. 



Fam. 5. Brisingidce. Arms long and rounded, sharply marked off from 

 the disc. Ambulacral grooves not reaching the mouth ; two rows of 

 ambulacral feet. 



Owing to the imperfect state of preservation in which the 

 remains of fossil Star-fishes are usually found, it is difficult 

 or impossible to speak definitely as to the precise affinities 

 of many of the extinct species. The Brisingidce are not cer- 

 tainly known to occur as fossils, and the Asterinidce are but 

 poorly represented ; the great majority of fossil Star-fishes 

 being thus referable to the families of the Asteriadce, the 

 Astropectinidce, and the Oreastridce. It should be borne in 

 mind, however, that the singular family of the Brisingidce, 

 forming an intermediate group between the Asteroidea and 

 the Ophiuroidea, may possibly prove to be of very ancient 

 origin, and to be represented by such Silurian and Devonian 

 types as Protaster, Eugaster, Tceniaster, &c., which we may 

 provisionally consider among the Opliiuroids. Lastly, as Star- 

 fishes are always of rare occurrence as fossils, and as they 

 are therefore of little importance to the general student, we 

 shall content ourselves here with simply glancing at the 

 more important types which have made their appearance in 

 the successive geological periods, briefly noting some of the 

 special characters of the more interesting ancient forms. 



In the Silurian period the genus Palceaster (figs. 142 and 

 143, B) is the most important. In this genus we have Star- 

 fishes in which the body is five-armed, the disc being very 

 small ; the ambulacral grooves are placed on the lower sur- 

 face of the arms, and are furnished with two rows of ambu- 

 lacral ossicles and pores, bordered on each side by a row of 

 " adambulacral plates," which are, in turn, bordered by a 

 series of " marginal plates." On the dorsal surface are three 

 or more rows of plates, which are stated to fit closely to- 

 gether, instead of forming pores by their junction. The 

 genus Palceaster comprises some species of considerable size, 

 and ranges from the Lower Silurian to the Devonian. The 

 Silurian genus Urasterella ( = Stenaster) is in many respects 

 like Palceaster, but the ambulacral grooves are margined by 



