MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 219 



Genus MEMBRANILARNACIA Eisenack 1963 : 99 

 Emended diagnosis. Chorate cysts spherical, oval or ellipsoidal, usually thick 

 walled and surrounded by more or less concentric and generally thin walled envelo- 

 ping membrane supported by processes or supports normal to central body. Proces- 

 ses forked, flared or branched distally. Shafts cylindrical or columnar. Archaeo- 

 pyle apical. 



Type species. Membranilamax leptoderma Cookson & Eisenack 1958. Lower 

 Cretaceous ; Papua. 



Discussion. Some of the species originally referred to the genus Membranilamax 

 O. Wetzel (1933) were transferred to Membranilamicia by Eisenack (19636). Eisen- 

 ack (1959c) reviewed Membranilamax and showed that the generic description 

 given by O. Wetzel was ambiguous, embodying three distinct groups. These are : 



1. Forms with a spherical to oval central body surrounded by a membrane 

 restricted to the equatorial region (pterate cysts). The membrane is 

 supported by processes arising from the central body and branching distally. 

 A pylome (or archaeopyle) has not been observed. 



2. Hystrichospheres with a spherical to oval central body surrounded by a 

 concentric outer membrane which is supported by processes arising from the 

 central body. A pylome (archaeopyle) can be present. 



3. Hystrichospheres with a central body surrounded by a concentric outer 

 membrane, supported by raised crests, which form polygonal fields on the 

 central body. A pylome can be present. 



To the first group can be attributed the type species of Membranilamax (M. 

 pterospermoides O. Wetzel). The genus should therefore be restricted to species 

 referable to group 1, but since the structure of the holotype of M. pterospermoides 

 cannot be clarified because the specimen is too deeply embedded in a flake of flint 

 and cannot be examined at high magnification, the placing of other species within 

 Membranilamax is not to be recommended. 



Eisenack (1963&) erected two new genera according with the second and third 

 types. These are Membranilamacia (corresponding to Group 2), and Valensiella 

 (corresponding to Group 3 and synonymous with Favilamax Sarjeant 1963c over 

 which it has seniority). Following Eisenack's abandonment of the name Membrani- 

 lamax, all residual species left in that genus were provisionally transferred to 

 Membranilamacia by Downie & Sarjeant (1964). Specimens from the London Clay 

 attributable to the genus Membranilamacia possess intratabular processes indicating 

 a reflected tabulation of 1-4', 6", 0-4C, 5'", ip, 1"". The archaeopyle is apical with 

 a zigzag margin. From the tabulation and possession of intratabular processes, 

 restricted to one per plate, Membranilamacia must be included in the family Hystri- 

 chosphaeridiaceae and cannot therefore be the type genus of the family Membrani- 

 larnacidiaceae. 



Membranilamacia is distinguished from the genus Adnatosphaeridium by having 

 an outer membrane instead of trabeculae, which distally unite the processes of the 

 latter genus. It is possible however that intermediate forms exist. 



