BLASTOIDEA. 35 



penetrate the interior, and no office has been conjectu rally 

 assigned to them; but Edward Forbes suggested that they 

 might represent the " epaulettes " of the larval Echinidce, to 

 which group he supposed the Cystidean bore the same relation 

 as the Crinoids hold to the star-fishes. Some Sphcerbriifes of 

 the Bala beds seem to have become freed from their stems, 

 and may have enjoyed a feeble locomotion ; the two genera, 

 Agelacrinus and Hemicystites, hitherto found stemless and 

 sessile on foreign bodies, are chiefly from the Silurian beds 

 of America. Of the known genera, of Gystoiclea, eight are 

 found in British strata — four in the upper and four in the 

 lower Silurian. 



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, composed of solid poly- 

 gonal plates, and supported on a small, jointed stalk, with 

 radiated articular surfaces and irregular side-arms. The 

 minute oral orifice is 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 Koemer, these supported numerous slender, jointed 

 tentacula, indicated by the rows of marginal pores. One 

 species 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 in it. 



As the star-fishes progress with their mouth downwards, 

 the side of the body on which it opens is called the " ventral," 

 the opposite side the " dorsal " surface ; and the same terms 

 are applicable to the homologous surfaces of the radiated disc 



