GENUS CERATIUM. 453 



A. — Horn-like Processes shorter than the Body. 

 Ceratium divergens, Ehr. sp. Pl. XXV. Figs. 8-13. 



Cuirass rhomboidal, widest centrally, attenuate and acuminate posteriorly, 

 the anterior extremity armed with two short, straight, pointed horns, each 

 of them having a tooth-like process at the basal portion of their inner 

 margin. Diameter 1-576". 



Hab. — Salt water : Baltic Sea. Colour yellow. 



This animalcule was first described by Ehrenberg under the title of Peridinimn 

 diyergeiis. Claparede and Lachmann regard the P. depresswn of Bailey (PI. XXV. 

 Figs. 12 and 13) as idendcal with this species. These same authorities, as shown in 

 the figures here reproduced, have observed the species in both its encysted and 

 naked or Gymnodinium phases of existence. The contour of the empty cuirass, as 

 shown at Figs. 8 and 9, presents a remarkably elegant facetted crystalline aspect. 



In the ' Monatsbericht der Berliner Akademie' for the year 1854 Ehrenberg has 

 described, under the title of P. divergens var. renifor7ne, a modification of the present 

 form characterized by a somewhat wider reniform contour of the cuirass, the posterior 

 extremity being further produced as a short, obtusely pointed, stylate process. 



Ceratium Michaelis, Ehr. sp. Pl. XXV. Fig. 23. 



Cuirass ovate, having two short, straight, anterior and one postero- 

 terminal, subcylindrical, horn-like processes, the extremities of which are 

 abruptly truncate; surface entirely smooth. Length 1-570". 



Hab. — Salt water. Highly phosphorescent ; colour yellow. 



The truncate termination of the horns, in addition to the absence of tooth-like 

 processes at the base of the two anterior ones, distinguishes this form from C. divergens^ 

 which it otherwise most nearly resembles. 



Ceratium tridens, Ehr. 



Cuirass resembling that of C. Michaelis and C. divergens, bearing three 

 acute frontal horns, the posterior extremity attenuate, surface granular. 

 Length 1-576". Hab. — Salt water : Baltic Sea. Colour yellow. 



Ceratium bicorne, Schmarda sp. Pl. XXV. Fig. 62. 

 Cuirass ovate or subspheroidal, having two anteriorly developed horn- 

 like processes only, the one taking the form of a straight, axial, acuminate 

 point, the second antero-lateral, curved, and spur-like, equal in length to 

 about one-half of the length of the body. Length 1-1080" to 1-840". 

 Colour brick red. Hab. — Salt water. 



Ceratium pyrophorum, Ehr. 



Cuirass ovate or subspherical, with two little acuminate points at the 

 anterior extremity ; surface delicately areolate and granular. Length 

 1-570" to 1-480". 



Hab. — Baltic Sea, near Kiel, and also as a fossil in the flints" of the 

 chalk formation from the neighbourhood of Brighton and Gravesend. 



