GENEllA AND &B0UP8 OP THE ECHINOIDEA. 19 



several vertical rows of plates and tlie ambulacra are narrow, and 

 with a multitude of small, low plates each perforated minutely by 

 a pair of pores ; as there are two pairs of pores on each side of 

 the median line and placed nearly horizontally, the presence ot' 

 two vertical rows of ambulacral plates on either side o£ the 

 median line must be admitted. The peristome presents some 

 appearance of jaws. There is not enough to define a new genus 

 from, and certainly the reason for placing the forms in JEchino- 

 cystites is not apparent. The alliance is with Oligoporus, but it 

 is necessary to wait for better specimens. (See Echinocystites.) 



Order III. PLESIOCIDAEOIDA. 



Test small, subhemispherical, solid, with a large apical system, 

 having large united basal plates and a central periproct. Ambu- 

 lacra narrow, and with two vertical rows of poriferous plates. 

 Interradia with a single peristomial plate, followed by three 

 plates separated by vertical sutures. Large tubercles on the 

 actinal surface. 



Grenus TiAEECHiNUS, Neumayr, 1881, Sitzungsb. d. Jcais. AJcad. 

 d. Wiss. Wien, 1882, Bd. 84, Heft i. p. 69. LovSn, Pourta- 

 lesia, 1883, Kongl. Svenska Vetensk.-Akad. Randl. Bd. xix. 

 pp. 11, 64, pi. xiii. 



Test small, flat actinally and slightly elliptical in marginal 

 outline, subhemispherical dorsally. 



Apical system very large, extending nearly halfway to the 

 ambitus ; periproctal space small, pentagonal; basals very large, 

 the posterior the smallest, hexagonal, forming a very broad ring; 

 genital pores two in number, one in the basals 1 and 3 ; radial 

 plates pentagonal, notching the union of the basals slightly ; pores 

 absent. 



Ambulacra band-like, equal, but the posterior pair are closer 

 together than the others ; straight, broadest at the peristome, 

 made up of two vertical rows of numerous low, broad primary 

 plates, each with a pair of pores and a small plain tubercle, 



Interradia broad, actinally composed of a single pei'istomial 

 plate, above which are three tall plates only, one median and the 

 others at its sides. Ornamentation, a plain primary tubercle 

 to each plate at the ambitus and actinally ; elsewhere the test 

 is coarsely granular, including the apical system. Sutures very 



2* 



