Tasch: Growth and Form of Fossil Protists 443 



Chrysomonadina are either solitary or colonial. They are widespread in both 

 fresh- ■'*^ and marine waters'^ and have fossil representatives in the family 

 Archaemonadaceae Deflandre.^" 



Formation of siliceous resting spores or cysts is a "most characteristic feature 

 of the order. "2 '18 '51 Such cysts have a funnel-shaped opening or neck and 

 resemble a stoppered or plugged spherical jar. The plug is formed of cyto- 

 plasm retracted from outside the cyst wall. 



In the cyst of Microglena,^^ Conrad has distinguished "numerous minute 

 lens-shaped masses of silica" embedded in an outer layer of pectic substances. 

 A delicate, inner smooth layer of cellulose underlies this outer layer. This 

 genus with other Chromulineae is closely related to the coccolithophorids in 

 cell structure although it differs from the latter in flagellation and composition 

 of its cyst. 



Cysts are usually spherical but variants from this configuration occur. 

 Archeomonadopsis, which is flask-shaped, is such a variant. Surficial orna- 

 mentation finds diverse expression: ridges that may form a reticular network; 

 encircling equatorial flanges; spine and knob structures on ridges, and peripheral 

 spines. The size range of cysts is 10 to 25 /x. 



Although little is known about the marine Chrysomonadina, it is apparent 

 that the morphology and small size of the cyst, together with the cytoplasmic 

 plug, would favor both resistance to sinking below the pelagic zone and wide- 

 spread passive distribution. Fossil cysts also indicate a broad geographical 

 spread.^" The same types of adaptive modification found in living representa- 

 tives occur in fossils. 



A third large group of planktonic algae are the Coccolithophoridae.'^'^^"' 

 52a ,53 ,54 fhey arc typically open sea biflagellates although in places like the 

 Oslo fjord, they may occasionally occur in such densities, that the water looks 

 like milk."* Fresh water forms like Hymenomonas are also known.^^ 



One may study a form like Coccochrysis,^^ Discosphaera,^ or Syracosphaera 

 and CoccoUlhus^^ and observe a subovate configuration in the first and third 

 and a more spherical form in the second and fourth. Lohman^- figures several 

 different species of Pontospliaera, Calyptrosp/iaera, and Coccolilhop/iora, as well 

 as species of the second and third genera named above. All of these species 

 show the same trend in configuration. Generally, therefore, the shape of 

 coccolithophorids are modifications of a sphere. 



The formation of the coccolithophorid skeleton is gradually achieved. At 

 fairly equal intervals, numerous, minute, variously shaped, calcareous discs 

 (coccoliths) are "imbedded in an investing membrane. "'^^ This envelope of 

 variable thickness is gelatinous initially. The coccoliths become "rigidly 

 united when the mucilage calcifies in older individuals."'** Coccoliths have a 

 central perforation or are imperforate."^ Although living biflagellates commonly 

 range from 5 to 20 n,'^ sizes can attain 50 )u.^^ Coccoliths found in sediments 

 range between 2 and 30 fj.}^ 



Several coccoliths bear anteriorly and medially spinelike processes. Of 

 interest, is the successive formation of new coccoliths within the old as the old 

 are gradually dislodged'^ and contribute to oceanic bottom deposits. Although 

 today coccoliths are but a "minor part" of oceanic carbonate muds, in Miocene 

 and Oligocene time, for example, they formed "coccolith ooze."^^ 



