PI. 



I, fig. 11. 



Length *625 



PI. 



IV, fig. 22. 



„ -7; 



PI. 



IV, fig. 25. 



•66; 



PI. 



IV, fig. 27. 



„ -7; 



PI. 



IV, fig. 28. 



„ -7; 



In 



some specimens from 



CYTHEROPTERON UMBONATUM. 41 



8*. Cytheeoptbeon umbonatqm {Williamson), var. acanthoptera (Marsson), (vel 



umhonatum-acantJiopterum). Plate I, 

 figs. 11—13; Plate IV, figs. 22—29. 



Cttheee acanthopteea, Marsson. Mittheil. naturw. Ver. Neu-Vorpommern und 



Eugen, 1880, p. 45, pi. iii, figs. 14 a — c. 



height '35 ; thickness (with spines) '55 mm. 

 •43; „ „ -66 „ 



„ -36; „ „ -66 „ 



,, '33 mm. 



.Q . •7Q 



55 ^ J 55 55 ' ^ 55 



from the Chalk of Horstead, and of Island Magee, 

 Ireland, we find the body of the valve much swollen, almost as much in front as 

 behind, and the edges of this tumid area more or less ridged and prickled (slightly 

 indicated in PI. I, fig. 12), quite coarsely and irregularly in an Irish specimen ; the 

 dorsal furrow is deep ; the antero-dorsal region sometimes bulges out into a low, 

 irregular tubercle (PI. IV, figs. 25 — 27) ; the postero-ventral spine is strong, and, 

 having a depression at its base, is, as it were, set in a notch in some specimens, 

 which have much stronger features than are shown in PI. I, figs. 11 — 13. The 

 punctation is coarse and mixed with prickles, mostly arising from the walls of 

 the meshes. The valves are broader (higher) than those of the typical form, and 

 vary in this feature as well as in the convexity of the ventral region. 



Dr. Marsson's acanthoptera is very like our figs. 11 — 13 of PL I, but is figured 

 with a smooth surface, and its medial depression reaches across the valve. These 

 discrepancies, however, are slight, and we think that they are not sufficiently 

 essential to separate the forms under notice. 



Localities. — Chalh, Horstead (Norfolk) and Island Magee (Antrim) ; Ghalk- 

 rocJc, Dunstable. 



Foreign. — Ghalk, Isle of Riigen. 



8**. Cytheeopteeon umbonatum (Williamson), var. longispinata, nov. (vel umbona- 



tum-longispinatum) . Plate III, figs. 

 11—13 ; Plate IV, figs. 30 and 31. 



Ctthebe tjmbonata, Marsson. Mitth. nat. Ver. Neu-Vorpommera und Eiigen, 



1880, p. 45, pi. iii, figs. 15 a—c. 



PL III, fig. 11. 

 PL IV, fig. 30. 



>• Length 1*0; height '53 ; thickness (with spines) 1*1 mm. 



