38 PALAEONTOLOGY. 



depth of two miles below the surface in the North Atlantic. 

 Trqpidast&r, Pleur aster, Aspidura, Ophiurella, and Amphiura 

 are oolitic genera ; Ophioderma, Luidia, Astropecten range 

 from the lias to the present seas ; Stellaster and Arthraster 

 are peculiar to the cretaceous ; and Ophiura, Ophiocoma, 

 Astrogonium, Oreaster, and Goniodiscus are both cretaceous 

 and living. 



Order 5. — Echinoidea. 

 (Sea-urchins.) 

 Char. — Body free, spheroid or discoid, incased in a crust of 

 inflexibly-jointed calcareous plates, and armed with 

 spines; mouth below, with a complex dental system, 

 usually arranged, so as to resemble a " lantern." 



The Echinoidea appear first in the Lower Ludlow limestone 

 and attain their maximum in the cretaceous strata. The 

 principal shell-plates are arranged in longitudinal series, five 

 of perforated or "ambulacral" (fig. 8, 8, a) alternating with 

 five of " inter-ambulacral " plates (ib. i). In all secondary 

 and more modern Echinidaz, each series includes a double row 

 of plates, which are pentagonal : but in the Silurian Palmo- 

 discus and EcJiinocyslites, the inter-ambulacral plates are of less 

 definite shape, and are crowded irregularly, so that from eight 

 to ten may extend transversely between the wider intervals 

 of the ambulacra ; and this low vegetative repetition of parts 

 is continued in the Perischodomus and PalcecMnus (fig. 8, i) 

 of the carboniferous limestone, where there are five or six 

 rows of plates in the inter-ambulacral areas. Only detached 

 plates of the equally ancient Archmocidaris have been seen, 

 and the inter-ambulacral ones (fig. 8, %), by their six-sided 

 form, seem also to have been arranged in more than two rows. 

 Normal Echinidse, of the existing genus Cidaris, abound in 

 the upper trios. Some of the secondary species of Cidaris 

 have the ambulacral pores widely separated ( = Rhabdoci- 



