20 PEOr. P. M. DUNCAN S BEYISION OF THE 



invisible; test solid-looking. Peristome large, oyal-elliptical, 

 without branchial incisions. 



Fossil. St. Cassian-Trias : Europe. 



It is to be obseryed that the single species of this genus, 

 T. princeps, Laube sp., was diagnosed from specimens 3"8 millim. 

 in height, 5'2 millim in length, and 4'9 millim. in breadth. 



Order IV. CTSTOCIDAEOIDA, Zittel, 1876-81. 

 Test irregular (exocyclic), globular or ovoid, thin, flexible (?) ; 

 madreporite central and dorsal. Ambulacra narrow, and with 

 two vertical rows of poriferous plates. Interradia broad, with 

 numerous vertical rows of scale-like moveable plates ; periproct 

 in the posterior interradium above the ambitus. 



There has been a difficulty made about the name of the 

 principal, if not the only, genus of this family. Wyville Thomson 

 defined the genus very well in 1861, and employed the term 

 Echinocystites, which was a good one. In 1864 Hall called a 

 genus of Cystidea by the name already occupied by Wy. Thomson's 

 genus. In 1876-80 Zittel, in his ' Palseontologie,' p. 480, altered 

 "W'y. Thomson's term to Cystocidaris, and noticed the fact about 

 Hall's using the na,me UcJiinocystites, although it was preoccupied. 

 Certainly Wy. Thomson's name must continue, and Hall's 

 Cystidean will have to be called by something else, and Cysto- 

 cidaris must lapse according to the ordinary rules of nomen- 

 clature. 



G-enus EcHiNOCTSTiTES, Wy. Thomson, 1861, Edinb. New Phil. 

 Journ. n. ser. vol. xiii. p. 108, pis. iii., iv. {non Echinocystites, 

 Kail, 1864). 



Syn. Cystocidaris, Zitt. ; Paleeo discus, Salt, in Wy. Thoms. op. 

 cit. p. 116*. 



Test large, spheroidal or ovoid, thin, flexible. 



Apical system central and dorsal, apparently consisting of a 

 large madreporite only. 



Ambulacra narrow, straight ; plates numerous, small, low, in 

 four vertical rows, a central pair of pores in each plate, and 



* The genus Falmodisous, Salter, 1857, Ann. & Mag. Nat. Hist. ser. 2 

 Tol. XX. p. 332, was considered by Wy. Thomson in the same essay as that 

 which contained the description of Echinocystites, and the soHtary species was 

 stated to be flexible and the teeth were as chisels. 



