1-- APPENDIX TO "STUDIES ON MESOZOIC 



a variable number of small plates. Rarely, an apical or an antapical pericoel may 

 be present (but not both) ; the two shell layers are otherwise in continuous contact. 

 Sutures typically marked by ridges or low crests (perforate or imperforate), without 

 spines or denticles. Height of crests always less (and typically markedly less) than 

 £ of shell width. A precingular single-plate archaeopyle, formed by loss of plate 3", 

 is developed, the operculum typically becoming wholly detached ; in some speci- 

 mens, the archaeopyle may not be developed. Surface of periphragm smooth, 

 granular, or punctate. Forms with nodose or reticulate surface have not been 

 encountered and forms with crest spines or with general spine cover are excluded. 



Type species. Leptodinium subtile Element, i960 ; 46-47, pi. 6 figs. 1-4, text- 

 figs. 23-24. Upper Jurassic (Kimmeridgian), Germany. 



Remarks. The diagnosis here formulated is an expansion of that given by Wall 

 (1967) : it differs in being more detailed and in permitting the inclusion of forms 

 showing differentiation of the ventral surface into plates. Species with an apical 

 horn are allocated to Gonyaulacysta ; species with sutures marked by lines of high 

 spines are placed in Hystrichogonyaulax gen. no v. ; species with a general spine 

 cover are placed in the genus Acanthaulax. The currently known range of the 

 genus Leptodinium, as here defined, is Upper Jurassic to Recent. 



Other species. 

 Leptodinium aceras (Eisenack) Sarjeant, comb, nov., =Gonyaulax aceras 



Eisenack, 1958 ; 391-2, pi. 21 figs. 1-2. Lower Cretaceous, Germany. 

 Leptodinium aculeatum Wall, 1967 ; 104-5, pi. 14 figs. 18-19, text-figs. 3C, 3D. 



Pleistocene — Recent, Yucatan Basin, Caribbean Sea. 

 Leptodinium alectrolophum Sarjeant, 1966b ; 134-5, pi. 15 figs. 3-6, text-fig. 34. 



Lower Cretaceous, England. 

 Leptodinium amabilis (Deflandre) Sarjeant, comb, nov., =Gonyaulax amabilis 



Deflandre, 1939 ; 143, pi. 6 fig. 8. Upper Jurassic, France. 

 Leptodinium arcuatum Element, i960 ; 48, pi. 6 figs 5-6. Upper Jurassic, 



Germany. 

 Leptodinium clathratum (Cookson & Eisenack) Sarjeant, comb, nov., =Gony- 



aulax clathrata Cookson & Eisenack, 1960b ; 246-7, pi. 37 fig. 5, text-fig. 2. 



Upper Jurassic, Australia. 

 }Leptodinium crassinervum (Deflandre) Sarjeant, comb, nov., =Palaeoperi- 



dinium crassinervum Deflandre, 1939 ; 144, pi. 6 fig. 5. (Transfer of this 



species to Gonyaulacysta was proposed by Sarjeant, 1967 ; 248-9). Upper 



Jurassic, France. 

 Leptodinium delicatum (Davey) Sarjeant, comb, nov., ^Gonyaulacysta delicata 



Davey, 1968a ; 123-4, pi- 1 > fig s - 7> 8, text-figs. ioA,B. Upper Cretaceous, 



Saskatchewan, Canada. 

 Leptodinium eumorphum (Cookson & Eisenack, 1960b ; 246, pi. 37 figs. 1-3, 



text-fig. 3) Eisenack, 1961 ; 324. Upper Jurassic, Australia. 

 Leptodinium freakei (Sarjeant) Sarjeant, comb, nov., =Gonyaulax freakei 



Sarjeant, 1963b ; 85-6, pi. 1 figs. 1-3. Upper Jurassic, England. 



