390 = G. C. Crick—Actinocamaz from the English Chalk. 
and transverse diameters being 11°5 and 10°5 mm. respectively, the 
greatest width being a little nearer the ventral than the dorsal 
surface. There are no traces either of a ventral fissure or of the 
alveolus. The dorso-lateral grooves are well-preserved; at the 
anterior end of the fossil they are rather broad, flattened, very 
slightly concave, about 6mm. apart, and each about 4mm. ‘wide ; 
proceeding from this end each narrows rather rapidly for about 
28 mm., being then only about 2 mm. wide; the two grooves separate 
rather quickly from each other for about one-half the length of the 
specimen; they are then continued nearly parallel to each other over 
another fourth of the length of the guard, when ve gradually 
disappear. 
Measurements. 
mm. 
Length - any OAs 
Dorso-ventral diameter at the most inflated part... we 13°75 
Transverse diameter at ditto bac oe a0 Beek). 
Dorso-ventral diameter at anterior end ... ath enamels) 
Transverse diameter at ditto ae ane SGORS 
Affinities and differences.—In a dorsal amet the specimen bears 
some resemblance to M. Janet’s figure (fig. 5a) of the dorsal surface 
of Actinocamax Alfridi,’ but that species is more lanceolate and has 
the posterior portion of its guard less inflated than the present 
specimen; whilst in a lateral ‘aspect the anterior half of the guard 
of this specimen tapers less rapidly than in that species. Compared 
with M. Janet’s figures of <Actinocamax Grossouvret,* the present 
' Bull. Soc. géol. France, sér. 11, tom. xix, No. 9 (Nov., 1891), pp. 720, 721, 
pl. xiv, fig. 5, and text-fig. 4. M. Janet does not give the dimensions of the single 
specimen upon which he based the species, but, taken from his figures, they are as 
follows : — Length, 96mm.; dorso-ventral diameter at the most inflated part, 
14mm. ; - transverse dinmaeter at ditto, 15mm.; dorso-ventral diameter at the 
deen oe part, which is at about the level of the ovisac, 9mm.; transverse diameter 
at ditto, 8 mm. 
2 Bull. Soc. géol. France, sér. 11, tom. xix, No. 9 (Nov., 1891), pp. 716-719, 
pl. xiv, figs. 1- 5, and text- figs. 2,3. For the sake of comparison with the forms 
described in the present paper, the dimensions of M. Janet’s co-types of Actinocamax 
Grossouvrei are here given :— 
it: Tal III. 
Length in millimetres... Bo Boo SIC ((E)) S71 91 
Dorso- ventral diameter at the most inflated part 009 soo LG 14 13 
Transverse diameter at the most inflated part . 20 18 15 
Dorso-ventral diameter at the narrowest ee nearly at the level 
of the ovisac . 50 soc 500 aoa Jae 8 § 
Transverse diameter at the same ‘place ey. ee &, balea 12 9 
To this species M. de Grossouvre (‘‘ Recherches sur 6 craie 5 STieaienne, ” pt. 1, 
fasc. 2, 1901, pp. 796 and 801) refers the unique specimen which M. Janet 
described and figured in the same paper (ibid., pp. 719, 720, pl. xiv, fig. 4, and 
text-fig. 1) as “Actinocamax Toucasi, the two forms being, according to M. de 
Grossouvre, extreme modifications of the same type and connected by a number of 
intermediate links. The dimensions of the holotype of M. Janet’s A. Toucasi are :— 
Length, 90mm.; dorso-ventral diameter at the most inflated part, 12mm. ; 
transverse diame at the most inflated part, 17mm. ; dorso-ventral diameter at the 
narrowest part at the alveolar extremity, 5mm. : transverse diameter at the same 
place, 6mm. Both forms occur at the same horizon as Actinocaman granulatus and 
Actinocamax verus; in France in strata which French authors regard as Upper 
Santonian and Lower Campanian (M. de Grossouvre, loc. cit.). It is interesting to. 
note that two similar modifications of this species have been recorded (as Actinocamax 
