148 



ECHINODERMA. CYSTIDEA 



having a rhombic form ; one half of each rhomb is on 

 one plate, the other on an adjoining plate (fig. 59 C). The 

 canals are parallel to the surface of the plates, and perpen- 

 dicular to the sutures between the plates. These groups of 

 canals are known as pore-rhombs. Echinosphcera, from the 

 Ordovician, is a form which possesses many pore-rhombs; it 

 has a spherical calyx, consist- 

 ing of numerous plates, some 

 of which project at the base 

 and probably served to fix the 

 calyx, there being no stem ; 

 around the mouth are from 

 three to five small arms. In 

 most genera belonging to this 

 group the plates of the calyx 

 are much fewer in number 

 than in Echinosphcera, and 

 have a distinctly radial sym- 

 metry — being arranged in 

 cycles, the plates of each 

 cycle alternating with those 

 immediately below ; for ex- 

 ample, the calyx of Lepado- 

 crinus, from the Silurian (fig. 

 GO), is formed of five cycles of 

 plates ; at the base is a cycle 

 of four plates, followed by four 

 cycles of five plates each; from 

 the summit of the ovoid calyx 

 four food-grooves stretch to- 

 ward the base ; they do not 

 rest directly on the calyx, as is the case in Glyptosphcera, 

 but on specially-developed plates. Numerous brachioles 



Fig. 60. Lepadocrinus quadri- 

 fasciatus, from the Wenlock 

 Limestone. Restored figure. 

 The brachioles of the outer rows 

 are erect ; those of the middle 

 row depressed. Near the top of 

 the left-hand quarter is the 

 anus ; near the top of the right- 

 hand quarter is a pectini- 

 rhomb. (From the Guide to the 

 Geol. Dept., Brit. Mus.) Natural 

 size. 



