210 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



Irish Sea Species of Peridinium. 



Mr. Riddell has drawn up for us the following list, 

 with diagnoses, of the species of Peridinium we have 

 found, so far, in our district. 



g. 3. 



plates ; 



Genus Peridinium. — Testa divided into 

 precingular plates seven, bottom plates two. 

 Sub-genus I. — Protoperidinium : Girdle nearer apex on 



right side; usually no antapical horns; spines may 



be present. 



(1) P. orbiculare, Paulsen ; globular, small ; no 



spines and no apical horn; girdle almost equa- 

 torial. L. 0*04 — 0'045 mm. Rare in Irish Sea. 

 (See fig. 3, No. 1.) 



(2) P. ovatum, Pouchet ; depressed ; apical horn 

 not prominent ; two antapical spines. L. 0062 

 mm., greatest breadth 0084 mm. Not un- 

 common. (Fig. 3, No. 2.) 



(3) P. steini, J org. ; pyriform; apical horn distinct; 



two antapical spines, each winged. L. 0045 — 

 0-052 mm. Pare. (Fig. 3, No. 3.) 



