ASTEEOIDEA 33 



Order 3. — Blastoidea. 



A separate order has been proposed for another small 

 group of palaeozoic fossils typified by Pentremites (fig. 6, 3). 

 The body is globular or elliptical, and supported on a small, 

 jointed stalk, with radiated articular surfaces and irregular 

 side-arms. It is composed of solid polygonal plates, with a 

 minute oral orifice at the summit surrounded by five other 

 openings, four of which are double and ovarian, the fifth rather 

 larger and anal. There are five petaloid ambulacra of variable 

 length, converging to the mouth, furrowed down the centre, 

 and striated across. According to the observations of Dr. 

 Ferdinand Bcemer, these supported numerous slender, jointed 

 tentacula, indicated by the rows of marginal pores. One spe- 

 cies is found in the upper Silurian, six in the Devonian, and 

 twenty-four in the Carboniferous, which has received the name 

 of "pentremite limestone" in the United States, on account of 

 the abundance of these fossils it contains. 



Order 4.— Asteroidea. 



(Sea-Stars, Brittle Stars.) 



Char. — Body radiate ; integument hardened by calcareous 

 pieces, and more or less armed with spines ; no dental 

 apparatus. 



Asteriadce and Ophmridm. — Fossil star-fishes, though less 

 common, have a wider range than their allies the fossil urchins, 

 being found amongst the earliest organic forms. Palazastcr, 

 Protastcr (fig. 7, 6), and Lcpidastcr (fig. 7, 5), are Silurian star- 

 fishes, presenting many anomalies, and scarcely referable to 

 any existing families. Trojridastcr, Pleuraster, Aspidura, Oplii- 

 urella, and Am/phiura are oolitic genera ; Ophiodcrma, Luidia, 

 Astwpcdcn range from the lias to the present seas ; Stcllaster 



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