128 MEMOIKS OF THE UNIVERSITY OF CALIFORNIA 



border of the hyi^oeone. In ^4.. Incustre (fig. U, 15) the V-shape trough extends 

 as a narrow line to the antapex. In four species only does it invade the epicone. 

 In A. corpulentum, A. vascuhim, A. scissHtn, and A. sulcatum (figs. U, 6, 27, 1, 

 26) it reaches near to and, in the last species, slightly beyond the apex. Its 

 borders posterior to the girdle are frequently extended in flaplike projections. 

 These are exaggerated on the left side in A. asymmetricum (fig. U, 5). In 

 A. sulcatum (figs. U, 10, 26) the right border is enlarged anteriorly, the left 

 border posteriorly. The antapex may be excavated by the suleal notch, as in 

 A. sulcatum and A. asymmetricum or smoothly roimded as in A. operculatum 

 (figs. U, 26, 5, 25). 



AmpJndinmm possesses the type of pusnle fomid throughout the Gymnodi- 

 niidae. One or two may be present, either opening anteriorly into the anterior 

 flagellar pore or posteriorly into the posterior pore. In A. tnmcatum these 

 are merged into one large, irregularly shaped pusule opening into both pores. 



The nucleus of AmpliicUninm usually has distinct, moniliform chromatin 

 threads and is without a perinuclear membrane. It is generally found near 

 the center or in the posterior part of the body. 



The genus Amphidinium shows a narrow range of coloring. Tlie predomi- 

 nant color is green, varying to greenish yellow, and yellow shading to orange 

 in ^1. cucurhita (pi. 1, fig. 9), and to yellow ochre in the other species. It is 

 probable that the brownish color given for many of the eai'lier described forms 

 is 3^ellow ochre, a common color throughout the Dinoflagellata. 



Striae or other surface markings are not so common in AmpJiidi)num as 

 in Gymnodiniiim and Gyrodinium. Three species, A. asymmetricum, A. den- 

 tatum, and A. scissum (fig. U, 5, 4, 1), show the fine surface striae common to 

 the latter genera. The surface in ^i. cucurhitella (fig. U, 30) and A. cucurhita 

 (fig. W. 3) is marked by deep furrows with very fine striations between, quite 

 unlike anything found elsewhere. In A. fastigium (fig. U, 11) it is marked by 

 a few distinct ridges and in .4. gaJhaiutm (fig. U, 29) by furrows. The surface 

 markings of ^l. hJehsi are too indefinite in Kleb's figure (1885) to be classified. 



Nutrition in the species of this genus is both holophytic and holozoic. When 

 chromatophores are present it is presmnably holoph}-tic, yet not invariably so. 

 In A. steini chromatophores are usually present, yet undoubted evidence of the 

 ingestion of foreign bodies is also given by Stein (1883, pi. 17, figs. 1-1-16). The 

 chromatophores in these figures are very small. Among our species, A. scissum 

 possesses what may be the remains of small chromatophores and also shows the 

 presence of two large food masses. The other forms without chromatophores 

 in most cases show the presence of food bodies within the cytoplasm. 



Cyst formation is common throughout the group. Binary fission may take 

 place within a cyst or in the freely swimming forms. 



DiSTBiBUTiox.— Species of AmpJiidinium have been described mainly from 

 the warm and cool temperate waters. None have been found thus far in tropical 

 seas and only one from the colder northern waters, ]\lielck (1911) reporting 



