7.6

91

272

263

62

178

6.9

100

95

EXPLANATION OP PLATE 22

Figs. 1, 6-10. Dimitobelus diptychus (M'Coy). 1, 6-8, right profiles and ventral outlines. 1, Albian-?Cenoman- ian, James Ross Island. D.8410.64, x I. 6, plaster cast of holotype, Albian, gueensland, Australia, NMV.P.2177, x l. 7, Albian, Alexander Island. KG.1717.7, x l. 8, Albian, South Australia.

GSSA,M.2476, x l. 9, silicone rubber cast, ventral outline, Albian, Alexander Island. KG.1748.16c, x 1.

10, juvenile, silicone rubber cast, (?)ventral outline, Albian, Alexander Island. KG.1748.15, x l.

Figs. 2-5. D, sp, nov. aff. diptychus (M'Coy). Campanian, James Ross Island, left profiles and ventral outlines.

2, D,8232.136A, x l. 3, D.8232.125E, x 1. 4, D.8232.136c. x l. 5, D.427.1A, x 1.

Figs. 11-15. D. stimulus Whitehouse. 11, specimen with pseudalveolus, left profile and ventral outline, Albian,

James Ross Island. D.8403.46, x 1. 12, holotype, right proSe and ventral outline, Albian, South Australia.

HM.S.5592, x l. 13, Silicone rubber cast, (?)ventral outline, Albian, Alexander Island. KG.1680.55, x l.

14, left profile and ventral outline, Albian, James Ross Island. D.8212.253, x 1, 15, right proSe and ventral outline, Albian, South Australia. HM.S.5590, x l.

Fig. 16. D. stimulus var. extremis Whitehouse. Albian, South Australia. Lectotype, right profile and ventral

outline. HM.S.5595, x l.

Description. Medium sized, subhastate Dimitobelus. The total length of the rostrum is approximately nine times Dv. The outline is symmetrical and subhastate, with Dlmax approximately at the mid-point, or nearer the apex of the rostrum. The apex is non-mucronate and acute. The profile is symmetrical and subhastate, with dorsum and venter slightly infiated. Transverse sections weakly depressed and almost perfectly elliptical, with no perceptible fiattening (Dlmax: Dvmax 1 2; text-fig. 6). However, the alveolar region may be more depressed,

Where present, two ventrolateral grooves extend for one quarter of the rostrum, but quite often the grooves are lost through formation of the pseudalveolus. The grooves curve dorsally to join the ventral-most line of the centrally positioned Doppellinicrt. The dorsal-most line may expand into a shallow, dorsolateral depression, but this is also lost in specimens possessing a pseudalveolus. The form of the phragmocone is ditIicult to assess due to the frequent development of a pseudalveolus and ¹delspitze in this species, but it apparently penetrates one quarter of the rostrum. The apical line is weakly cyrtolineate, and an apical canal is common.

Discussion. Dirnitobelus stimulus commonly occurs in some numbers with D. diptychus (M'Coy) in the Australian Great Artesian Basin, and is here described for the first time outside Australia.