SPATANGOIDEA 



557 



vertical bands of plates composing the corona is variable, in a 

 word, that the corona has not yet acquired a fixed definite con- 

 stitution. One genus {Echinocystites) has the anus outside the 

 apical system. It has four rows of pore-plates in each radius, 

 and numerous rows of plates each with a single spine in the 

 interradii. Another {Palaeodiscus) has been shown by Sollas^ to 

 be in many respects the missing link between Asteroidea and 

 Echinoidea. Inside the plates of the corona there is a series of 

 ambulacral plates like those of 

 Asteroidea. The tube-feet in the 

 oral portion of the radii seem to 

 have issued between the (outer) 

 ambulacral plates. 'So anus has 

 been detected. All the rest are 

 Endocyclic. The oldest known 

 form, Bothriocidans (Fig. 252, 

 A), from the Ordovician, has only 

 one row of interambulacral plates 

 and two of ambulacral ; no peri- 

 stome is distinguishable from the 

 corona. The Archaeocidaridae 

 appear in the Devonian. They 

 have narrow ambulacra of two 

 rows of pore -plates as in the 

 Cidaridae, but the interambu- 

 lacra consist of many rows, the 

 members of which overlap, and 

 therefore were probably slightly 

 movable, as in the Echinothu- 

 riidae ; the primary tubercles are large, and there is only one 

 on each plate. The Melonitidae (Fig. 252, B) appear in the 

 Carboniferous. Each interambulacral plate, of which there may 

 be five rows in each interradius, bears numerous small tubercles, 

 and there may be four or more vertical rows of pore-plates, though 

 in the genus figured, Falaeoechinus, there are only two. The 

 Tiarechioidae are represented by one genus, Tiarechinus, with an 

 enormous apical system, from the Triassic of the Tyrol. The 



,jn)l> 



-inter 



Fig. 252. — A, Bothriocidaris. x ] . B, 

 PalaeoecMnus, x 1. ainl}, Ambulacral 

 plates ; inter, interambulacral plates. 

 (After Zittel.) 



iterambulacra consist of one plate bordering the mouth, three, 



1 "On Silurian Echinoidea and Ophiuroidea," Quart. Journ. Geol. Soc. Iv. 

 1899, pp. 701 et seq. 



