12 Trans. Acad, Sci. of St. Louis. 



Apical disk composed of five or ten plates, which may or may 

 not be perforated. In all, except in Echinocystis, the anus 

 is central and surrounded by the apical disk. 



A. CYSTOCIDAEOIDA Zittel 1879.* 



Test composed of thin, spine-bearing plates. Ambulacral 

 area consists of two or four columns of plates, each plate 

 pierced by a central pair of pores. Interambulacrum broad, 

 composed of numerous columns of irregular, scale-like plates, 

 each plate bearing a small primary tubercle, surrounded by a 

 scrobicule. Anus aboral, in Echinocystis in the posterior 

 interambulacrum. Peristome small with highly developed 

 jaws. Mouth central. No apical plates. 



I. Palaeodiscidae Gregory 189 7. t 



Body depressed, discoid. Ambulacra composed of two 

 columns of plates crowded together. On the oral surface they 

 are imperforate, while near the aboral pole pores occur. Anus 

 central. 



a. Palaeodiscus Salter 1857. J 



Body discoid, test thin, flexible. Ambulacra broad with a 

 series of single pores on each side on the aboral surface. On 

 the oral surface, the ambulacral plates are simple, thin and 

 bar-shaped. On the aboral surface there are two alternate 

 series of simple, thin, bar-shaped plates. The interambulac- 

 ral plates are angular, irregular and scale-like, covered with 

 numerous short spines. Apical plates not known. 



1. Palaeodiscus ferox Salter. 



1857. Palaeodiscus ferox. Salter, Ann. Mag. Nat. Hist., 

 ser. 2, vol. 20, p. 332, pi. 9, fig. 6. 



♦ Handbuch der P^laeontologie Bd. 1, p. 480. 



t Quart. Journ. Geol. Soc. London, vol. 63, pt. 1, p. 133. 



X Ann. Mag. Nat. Hist., ser. 2, voL 20, p. 332. 



