28 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



IV. THE GENERA HYSTRICHOSPH A ERA AND A CHOMOSPH A ER A 



By R. J. DAVEY & G. L. WILLIAMS 



INTRODUCTION 



The celebrated German microscopist, C. G. Ehrenberg, was the first to notice the 

 occurrence of minute spiny organisms in flakes of Upper Cretaceous flint. These 

 organisms were divided by him into two types. The first type possessed oval to 

 polygonal shells bearing numerous forked processes and characterized by two 

 furrows, one encircling the shell and the other perpendicular to it on one surface 

 only. Such forms he recognized as belonging to a group of present-day plankton, 

 the dinoflagellates. The second type had spherical or oval shells bearing forked 

 processes as before but not possessing furrows. These forms he found rather 

 difficult to identify, but came to the conclusion that they were silicified zygospores 

 of a freshwater desmid known as Xanthidium. His initial findings were published in 

 1838 and 1843. 



In 1838 Ehrenberg came to England and visited the Clapham Microscopical 

 Society where he greatly influenced a group of British microscopists — Mantell, 

 Reade, Deane, White and Wilkinson. Mantell (1845), after critically examining the 

 shells of the Xanthidia, came to the conclusion that they were composed of some 

 flexible substance, probably organic, perhaps chitin or cutin. Later (1850) he 

 suggested that the spiny spheres were " probably the gemmules of sponges or other 

 zoophytes ", and proposed the new genus Spiniferites to include them. This new 

 name, however, was overlooked by subsequent workers and was eventually abandoned 

 as a nomen nudum (Sarjeant 1964). 



In 1904 the German marine biologist, Lohmann, after having worked on modern 

 plankton and examined the fossil spiny spheres, decided that the latter were definitely 

 planktonic. He came to the conclusion that they were eggs of a marine crustacean, 

 probably a Copepod, and for this reason gave them the name Ova hispida. Reinsch 

 (1905), for the first time, considered these fossils to be the cysts of marine algae, 

 possibly dinoflagellates. He termed them " palinospheres ", another name which 

 never came into general use. 



O. Wetzel (1933) rejected all previous attributions and placed them in a new 

 family, Hystrichosphaeridae, of unspecified systematic position. All the described 

 species were included in his new genus Hystrichosphaera, and Hystrichosphaera 

 furcata and H. ramosa were designated as joint type species. 



Deflandre (1937) emended Wetzel's genus Hystrichosphaera to include only those 

 forms possessing an equatorial girdle and polygonal fields. His choice of H. furcata 

 as the sole type species has, however, proved an unfortunate one. Those forms 

 without surface ornamentation he placed in a new genus, Hystrichosphaeridium 

 Deflandre. 



Evitt (1961) considered that Hystrichosphaera was not a motile dinoflagellate but 

 a cyst possessing structures which are reflections of features seen in the motile stage. 

 The presence of a precingular archaeopyle was noted and compared with that present 



