Oh. XXV.] 



MOUNTAIN" LIMESTONE. 



405 



Of the Bryozoa, the prevailing forms are Fenestella and Polypora, and 

 these often form considerable beds. Their net-like fronds are easily rec- 

 ognized. 



Crinoidea are also numerous in the Mountain Limestone. (See figs. 

 518, 519.) 



Fig. 51S. 



Fig. 519. 



Cyathoerinites planus, 

 Miller. Body and 

 arms. Mountain 

 Limestone. 



Cyathocrinus caryocrinoid.es, M'Coy. 

 a. Surface of one of the joints of the stem. 

 6. Pelvis or hody ; called also calyx or cup. 

 e. One of the pelvic plates. 



In the greater part of them, the cup or pelvis, fig. 519 b, is greatly- 

 developed in size in proportion to the arms, although this is not the case 

 in fig. 518. The genera Poteriocrinus, Cyathocrinus, Pentremites, 

 Actinocrinus, and Platycrinus, are all of them characteristic of this 

 formation. Other-Echinoderms are rare, a few Sea-Urchins only being 

 known : these have a complex structure, with many more plates on their 

 surface than are seen in the modern genera of the same group. One 

 genus, the Palcechinus (fig. 520) is the analogue of the modern Echinus. 

 The other, Archceocidaris, represents, in like manner, the Cidaris of the 

 present seas. 



Of Mollusca the Brachiopoda (or Palliobranchiates) constitute the 

 larger part, and are not only numerous, but often of large size. Perhaps 

 the most characteristic shells of the formation are large species of Pro- 

 ductus, such as P. giganteus, P. hernisphcericus, P. semireticulatus (fig. 

 521), and P. scabriculus. Large plaited spirifers, as Spirifer striatus, 



Fig. 521. 



Fig 520. 



^aUhdhinus ffiffas, M'Coy. Eeduced. 



Mountain Limestone: 



Ireland. 



Productus semireticulatus, Martin, sp. 

 (P. antiquatus. Sow.) Mountain 

 Limestone. England ; Russia ; the 

 Andes, &c. 



