200 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Holotype. B.M.(N.H.) slide ¥.51729(1). Speeton Clay, Shell West Heslerton 

 Borehole at 39 metres depth, West Heslerton, Yorks. Lower Cretaceous (Middle 

 Barremian). 



Dimensions. Holotype — overall length of enclosing body go[i, breadth 53— 5[x, 

 length of apical horn i8[i., of antapical horn 5[x ; overall length of inner body 67-5^1, 

 breadth 38-5[x. Range of dimensions ; overall length of enclosing body c. 70-95^. 



Description. The holotype is enclosed in a spindle-shaped cloak of debris : in 

 other specimens, the debris cloak appears less well formed. 



The outer membrane is spindle-shaped, with unequally developed horns. In the 

 holotype, the apical horn tapers sharply to an acute point from about mid-length : 

 this was not the case in other specimens, where the apical horn tapered more smoothly 

 from base to tip. The inner body is of comparable shape, its apical horn always 

 tapering smoothly from base to tip. 



No specimen seen shows a well-developed archaeopyle ; however, the holotype 

 shows a slit, corresponding in position to a precingular opening, which may be an 

 archaeopyle that has either incompletely opened or subsequently closed up. 



Fig. 51. Netrelytron trinetron sp. nov., showing the shell surrounded by the 

 enclosing cloak of organic matter. x c. 500. 



