ECHINODERMATA : DISTRIBUTION. 223 



maximum in the Palaeozoic period, from which time they have 

 gradually diminished down to the present day. On the other 

 hand, the free Crinoids are comparatively modern, and seem 

 to have reached their maximum at the present day. As has 

 already been described, the Palaeozoic Crinoidea differ in some 

 important particulars from those which succeeded them. The 

 order is well represented in the Silurian, Devonian, and Car- 

 boniferous rocks, but especially in the latter; many Carbon- 

 iferous limestones (crinoidal limestones and entrochal marbles) 

 being almost entirely made up of the columns and separate 

 joints of Crinoids. In the Secondary rocks Crinoids are still 

 abundant. In the Trias the beautiful "Stone-lily" (Encrlnus 

 liliiformis) is peculiar to its middle division (Muschelkalk). 

 In the Jurassic period occur many species of Apiocrinns (Pear- 

 encrinite), Pentacrinus, and Extracrinus. The Chalk also 

 abounds in Crinoids, amongst which is a remarkable unat- 

 tached form (the Tortoise-encrinite or Marsupites}. 



Of the non-pediculate Crinoidea, which are a decided ad- 

 vance upon the' stalked forms, there are comparatively few 

 traces ; but remains of forms (such as Saccosoma and Solano- 

 crimts) allied to the recent Comatiilcz have been found in the 

 Jurassic and Cretaceous deposits. 



2. BLASTOIDEA. The Blastoidea, or Pentremites, are entirely 

 Palaeozoic, and attain their maximum in the Carboniferous 

 rocks, some beds of which in America are known as the Pen- 

 tremite limestone, from the abundance of these organisms. 

 They are, however, also found in the Silurian and Devonian 

 rocks. 



3. CYSTOIDEA. These, like the preceding, are entirely Palae- 

 ozoic ; but they are, as far as is yet known, almost exclusively 

 confined to the Upper Cambrian and Silurian rocks, being 

 especially characteristic of the horizon of the Bala limestone. 

 The last known forms of Cystideans occur in the Devonian 

 rocks. The oldest known Echinoderms are two extremely 

 simple Cystideans (Trochocystites and Eocystites) which have 

 been discovered in the primordial zone of North America. 



4. ASTEROIDEA. These have a very long range in time, 

 extending from the Lower Silurian period up to tho present 

 day. In the Silurian rocks the genera Palceaster, Stenaster, 

 Palceodiscits , and Petraster are among the more important, the 

 greater number of forms being Upper Silurian. The next 

 period in which Star -fishes more especially abound is the 

 Oolitic (Mesozoic) ; the more important genera being Uraster, 

 Luidia, Astropecten, Plumaster, and Goniaster, some of which 

 have survived to the present day. Many Star-fishes occur, also, 



