43° 



HISTORICAL GEOLOGY 



connected by vertical rods. Limestone masses, sometimes five by 

 ten feet in horizontal extent and several inches thick, were built 

 by them. The aggregate amount of limestone built by the stroma- 

 toporoids was very large. 



Corals. — This class was present in the Ordovician and was rep- 

 resented by several types, among which were the simple, horn-shaped 

 cup corals (Fig. 399 B) and those living in colonies (Fig. 399 A). 

 The description of these types will be taken up under the Silurian 



(p. 444). 



Echinodermata 



Cystoids (Greek, custis, a bladder) were so named because of the 

 bladder-like shape of the body. Essentially, the animals had a sack- 

 like or bladder-like body made up of calcareous plates, on the upper 



A , ■ ^ 



Fig. 400. — Ordovician cystoids: A, Lepidodiscus {Agelacrinus) cincinnatiensis ; 

 B, Pleurocystis filitextus ; C, Amygdalocysiites florealis. 



side of which two arms were sometimes attached, while some species 

 were armless (Fig. 400 A-C). The body was rooted by a tapering 

 stem to the sea bottom. Cystoids first appeared in the Cambrian 

 and reached their climax in the Ordovician and Silurian, after which 

 they suddenly diminished in the number of species, although locally 

 a few forms lived in considerable abundance. They are characteristic 

 of the Ordovician and probably became extinct early in the Carbon- 

 iferous. 



Crinoids (Greek, crinon, a lily) are living in the present seas and 

 still constitute a vigorous stock, even though the race began in the 

 Cambrian. The name " sea lily " was given to this class of animals 

 because of their flower-like appearance. The animal (Fig. 401 A-D) 

 consists of a body composed of plates, as in cystoids, and is attached 

 to the sea bottom by a jointed stem. From the upper margin of the 

 body (calyx) spring the arms, which are short and simple in some 



