KEVISION" OF PALEOZOIC STELLEEOIDEA. 49 



Palxaster; it is also well developed though less conspicuous in Meso- 

 palseaster, Spaniaster, Xenaster, Agalmaster, Devonaster, Neopalxaster, 

 Palasterina, Monaster, Calliasterella, and Urasterella. 



As the disk plates are rarely well preserved, and as the centro- 

 dorsal has been seen in many Paleozoic species, the -writer thinks 

 it will be found in most of the forms of this era. 



Primary dlsJc plates. — In the most primitive and oldest Paleozoic 

 asterids the centro-dorsal is surrounded by a variable but small 

 number of diminutive accessory disk pieces. In mature Hudsonaster 

 they form a single ring, but in the young of the cryptozonian Uras- 

 terella ( U. vlridii) and in mature Calliasterella there are none of these 

 accessory pieces present. For these reasons it is thought that in 

 the Ordovicic there will be found a small asterid, even more primitive 

 than Hudsonaster, that will be devoid of accessory disk pieces. 



Around the centro-doi'sal of young Urasterella and in mature 

 Calliasterella, there foUows directly a rmg of five larger plates. The 

 same five plates are also seen outside of the single ring of accessory 

 pieces in Hudsonaster and Spaniaster, and directly beyond the two 

 rings of accessories in Mesopalseaster. These more or less large 

 plates are radial in position and are the basal radialia above which 

 contmue the columns of successively formed radials. 



Returning to Hudsonaster, we see that the firet rmg of promment 

 plates has 10 pieces, 5 of which are radial in position, the remainder 

 interradial. The same is true for Spaniaster and Xenaster, while 

 Calliasterella has an equal number of plates in the second ring. An 

 analysis of these disks shows that 5 of the plates a,re either primary 

 or secondary radialia, while the remainder are situated interradially, 

 and upon them rest the 10 columns of supramarginals. The first 

 prominent ring in Mesopalxaster sha^eri and Devonaster and the 

 second rmg in Urasterella nlricM each have 15 plates, 5 of wliich 

 are radialia, while the other 10 are supramai^mals. Simplifying 

 these statements and cuttmg out the accessory pieces, we learn that 

 the disks of these genera — and others could bo added — ^are composed 

 of a centro-dorsal, a first ring of 5 radialia, and a second one with 5 

 radialia and 10 basal supramarginaiia. This, then, is the same struc- 

 ture postulated for the hypothetic phylembryo of Asteroidea and 

 deduced through reduction of ossicles, as gleaned from a partial 

 ontogenesis of Hudsonaster. 



Axillary ossicles. — In the most piimitive asterid, Hudsonaster, one 

 sees in each axilla a single large plate, on either side of which adjoin 

 the 10 columns of inframarginals. The same arrangement obtains 

 in Siluraster, Palseastcr, and Australaster (here the genus occurs in the 

 Permo-Carboniferous, and the axillaries arc very lai^e contrasting 

 with the smaller ones of the Ordovicic Hudsonaster). In no other 

 Paleozoic genera docs this same development hold, though the single 



