MESOZOIC AND CAINOZOIC DINOFL AGELLATE CYSTS 63 



Derivation of name. Latin, costa, rib — referring to the thickenings of the 

 periphragm. 



Diagnosis. Subspherical central body possessing a moderate number of fibrous 

 tubiform processes having sub-quadrate distal openings. Distal margin denticulate 

 with small number of recurved prolongations. Thickened ribs extending from some 

 of the angles of the distal margins along length of processes and onto central body 

 surface where they connect with similar ribs extending from neighbouring processes, 

 forming mesh-like structure. Apical archaeopyle usually present. 



Holotype. B.M.(N.H.) slide V.51708, from G.R. 053890, Scarborough Castle, 

 Yorkshire. Oxford Clay (Quenstedtoceras mariae Zone). 



Dimensions. Holotype : diameter of central body 47 by 47^, length of processes 

 19-28(1, number of processes 24. 



Description. The surface (periphragm) of the central body is granular or 

 slightly reticulate. The tubiform processes have broad bases, and splay out distally 

 in an angular fashion, their width not exceeding c»jx medially. At the angles of the 

 margin are commonly situated small prolongations or spines, occasionally up to 8jjl 

 long but usually considerably smaller. The processes vary considerably in width, 

 from 1 to o,fx when measured medially. Only one or two finer processes are present 

 on a specimen and these are probably sulcal processes. An occasional deeply 

 furcate process has been observed in the central region of the cyst, these undoubtedly 

 being cingulum processes. The ribs (thickenings of the periphragm) are very 

 characteristic and form upon the surface of the central body a subpolygonal mesh- 

 like arrangement, somewhat simulating the tabulation seen in the genus Hystri- 

 chosphaera, but certainly not analogous to it. An apical archaeopyle is usually 

 present, and the reflected tabulation appears to be that typical for this genus. 

 Hystrichosphaeridium costatum sp. nov. has been recorded from the Oxfordian of 

 England, France and Germany. 



Remarks. Deflandre (1938) described H. salpingophorum from the Oxfordian 

 noting the ribbed processes and an apparent similarity to the Upper Cretaceous form 

 of this species. However, ribbing on the surface of the central body was not 

 remarked on or shown in his illustrations (pi. io, figs. 1-3). These forms possibly 

 belong to H. costatum. 



Klement (i960) described and figured specimens identical to those found in the 

 British Oxfordian, possessing the characteristic thickenings of the periphragm. 



The specimen illustrated by Lejeune-Carpentier (1940, text-fig. 7) from the Upper 

 Cretaceous, has vague polygonal markings on the surface of the central body, but 

 definite ribs are absent. 



Sarjeant (i960, 1961) illustrated specimens of H. costatum as H. salpingophorum ; 

 his figured specimen (1961, pi. 15, fig. 7) has been selected as holotype of the new 

 species. 



