40 



PROTOZOA 



E. lima (Ehrenberg) (Fig. 54). Anterior border of both shells 

 slightly indented; length .04 mm.; slow of movement: at Woods Hole. 



ORDER 2. DINIFERIDA. 



Two grooves present, a transverse and a longitudinal: 2 families. 

 Key to the families of Diniferida: 



! Transverse groove near middle of body 1. PEBIDINIDAE 



o 2 Transverse groove above the middle 2. DINOPHYSIDAE 



FAMILY 1. PERIDINIDAE. 



Transverse groove medium; longitudinal groove short; shell, when 

 present, composed of plates or not; plates either equatorial (bordering 

 the transverse furrow), apical, or antiapical, while a rhombic plate may 

 extend from the transverse furrow to the apex: about 4 genera. 



Key to the genera of Peridinidae: 



a : With shell. 



6 X Reticular markings on shell. 

 c x Anterior part of shell with 7 equatorial and 1 rhombic plates. 



1. PERIDINIUM 

 C 2 Anterior part of shell with 3 equatorial and no rhombic plates. 



2. CEBATIUM 



6 2 No markings on shell 3. GLENODINIUM 



c 2 Without shell 4. GYMNODINIUM 



1. PERIDINIUM* Ehrenberg. Body globular or elongate; shell with 

 distinct transverse groove, which may be spiral and with about 20 

 plates: 9 species, fresh and salt water; many species are reddish in 

 color and may be in sufficient numbers to color the sea. 

 P. digitale Pouchet (Fig. 55). Shell with large 

 pits and with oblique furrow, 1 posterior and 2 

 anterior spines; length .06 mm.: Woods 

 Hole; marine; common. 



P. divergens Ehr. Shell spherical, 

 tapering posteriorly, with 2 large spines 

 anteriorly ; length .07 mm. : Woods Hole ; 

 marine; common. 



2. CERATiUMf Schrank. Body a 

 flattened sphere with 3 long projections; 



Fig. r>4 Exuviella transverse groove either spiral or cir- 

 lima (Calkins). , , -La* i 



cular; longitudinal groove usually wide; 



shell reticulate or striped and composed of 10 plates ; color usually green 

 or brown: numerous species; in fresh and salt water. 



* See "Peridinium and the 'Red Water' in Narragansett Bay," by A. D. Mead, 

 Sci. N. S., Vol. 8, p. 707, 1898. 



t See "Mutations in Ceratium," by C. A. Kofoid, Bull. Mus. Comp. Zool., Vol. 52, 

 p. 213, 1909. 



Fig. 55 



Peridinium 



digitale 

 (Calkins). 



