CRETACEOUS ECHINODERMATA. 327 
arms the plates are modified to form an eyelid or protecting apparatus for the 
rudimentary eye which is at the end of each. The centre of the under surface of 
the disc presents the mouth with its protecting apparatus. In the cavity are the 
viscera of digestion; from the mouth run as many long grooves as there are 
arms ; they extend to the ends of the latter. These grooves are bordered by pe- 
culiar plates (ambulacral plates or ossicula), and mark the course of the aquiferous 
system of vessels, which in the living animal supply and inject with fluid the 
contractile tubes or suckers which fill the grooves, and by means of which the 
creature walks. In the interior of the arms extend the ramified ovaries, which 
have their openings in the interspaces. The surface of the under disc not oc- 
cupied by the ambulacral plates is supported by ossicula, similar, but not quite 
of the same form, with those of the upper surface. 
These arrangements are somewhat different in the Ophiuride or brittle stars, 
In this tribe the ovaries do not extend into the arms; the disc is round and 
shaped like a piece of money. It is usually covered with plates and spines, and 
presents a pair of peculiar plates opposite the origin of each of the five arms; 
the latter are slender and snake-like, and extend to considerable distances from 
the body. In the living state the Brittle-stars are very active animals, but very 
fragile and irritable, casting away and breaking into pieces their arms whenever 
laid hold of. 
Genus Oreaster, Miiller and Troschel. 
Very convex, more or less stellato-pentagonal star-fishes, having strongly con- 
structed skeletons, often bearing on the disc large tubercles or spines. Two 
rows of large plates form the borders. 
This genus is synonymous with Pentaceros of Linck, but that name is objection- 
able, since it has been applied to a genus of fishes by Cuvier and Valenciennes. It 
formed part of Goniaster as defined by Agassiz. The fossil species are found in 
the upper and lower chalk, and some of them bear considerable affinity to exist- 
ing forms. Those now living are all tropical, and often of very great size. 
Oreaster coronatus. (Tab. XXI. fig. 7, 74,76, 7c,7d.) R. 4. 
Body pentagonal with prolonged arms ; disc thick, convex, and coronated with 
a circle of large, more or less polygonal nodose pyramidal tubercles (spines). 
Ossicles of disc variable in size and very irregular in shape ; those towards the 
centre for the most part largest. 
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