KOPOID AND SWEZY: UNARMORED DINOFLAGELLATA 125 



Amphidinium, Delage and Herouard (1896), p. 386, fig. 678. 



Amphidinium, Sehonichen and Kalbeiiah (1900), p. 230, pi. 8, fig. 2; (1909), p. 251, pi. 



8, fig. 2. 

 Amphidinium, Calkins (1902), p. 432, fig. 27. 

 Amphidinium, Paulsen (1908), p. 95. 

 Amphidinium, Lohmann (1909), p. 223, figs. 19-21, 26. 

 Amphidinium, Lemnierniann (1910), pp. 580, 615, figs. 1-8. 

 Am-phidinium., Scliilling (1913), p. 13, fig. 9. 



Diagnosis. — Body usuall,v conii)i'essed ; girdle placed far anteriorly, with 

 little or no displacement; sulcus extending from girdle, rarely from apex, to 

 antapex without torsion ; epicone relatively minute. Nucleus generally poster- 

 iorly or centrally located. Pusules opening anteriorly into the anterior flagellar 

 pore, posteriorly into the posterior pore, or both may lie fused into one. No 

 nematocysts present. Plasma colorless with or without chromatophores or 

 colored without chromatophores. Surface may show fine striae, furrows, or 

 ridges may be smooth. Encystment in thin-walled membrane frequent. Nutri- 

 tion holophytic or holozoic. Length, 12 to 110/^. Marine, fresh and brackish 

 waters, eupelagic and neritic forms; warm or cold temperate seas; 22 species 

 described. 



Organology 



The genus Amphidinium is, in many respects, the most simply organized of 

 the Gymnodiniidae, yet it anticipates the conditions found in the more highly 

 evolved groups even of the Dinoph_ysidae. The body is without torsion and 

 differs from Gijmnodiiiivm mainly in the position of the girdle, the greater 

 frequency of a dorsoventral compression of the body and the more frequent 

 possession of chromatophores. 



The girdle is placed far anteriorly, at least on the dorsal side, and either 

 forms a circle transversely aroimd the l)ody or turns posteriorly on the ventral 

 side. One species only, A. scissiini (fig. U, 1), has a disi^lacemeut of the girdle 

 similar to that found in Gi/iiiiioflinimn and Gi/rodiniiiiii. In the other species 

 the ends of the girdle meet without displacement, the apparent exception being 

 A. sulcatum (fig. U, 10). In this species the girdle becomes contracted on the 

 right side, while the left side is expanded, foi'uiiug an a]i])arent dis))laceuient 

 of the proximal borders but not of the distal borders of tlie girdU; (fig. U, 2()). 



The anterior position of the girdle makes the relative sizes of the epicone and 

 hypocone one of the most striking features of the genus. The divergent form, 

 A. asymmetricum, (fig. U, 5), shows the greatest deviation from the type in this 

 respect, and foreshadows the conditions in G nmnndinium. The position of the 

 girdle dorsad, however, is farther anterior than is the case in any species of that 

 genus. 



The larger group of species (subgenus Amphidiiiiuiii sensu strictu) shows 

 a strong dorsoventral compression of the body with one exception, A. sulcatum, 



