CKICK : ON NAUTILUS JIoKATTAMESSIS, FOOED. 289 



Chambers moderately deep, about two-ninths of the diameter of the 

 shell in depth at the median line of the peripbery, about sixteen in 

 a whorl. Septa moderately concave, their dorsal (inner) margin 

 projected forward. Siphiincle suh-dorsan in the nepionic stage, but 

 gradually becoming median in the ephebic (?) stage. Suture-line 

 Avith a sinus on the umbilical zone, a well-marked saddle on the 

 outer side of the umbilical margin, a feeble sinus on the middle of 

 the lateral area, a broad low saddle on the peripheral margin, an 

 exceedingl)' shallow sinus on the peripheral ai"ea, and no dorsal 

 (annular or columellar) lobe. Test not seen. 



Thougli apparently closely related to W Archiac & Jiaime' s JVauttlus 

 forbesi,^ of which the type-specimen came from the Eocene of Sind, 

 India, that species is not only, as Dr. Foord pointed out, a " much 

 narrower and more compressed shell ", but compared vpith the 

 Egyptian form its siphuncle is nearer the dorsal (or inner) edge of 

 the septum. From Nautilus imperialis,'^ to which the Egyptian form 

 has a considerable resemblance, and which has already been recorded^ 

 from the Mokattam Range, Nautilus mokattamensis is distinguished by 

 its relatively greater thickness, the more nearly median position of 

 its siphuncle, and the greater slope of the outer side of the saddle 

 situated near the umbilical margin. 



' Le Vicomte d'Archiac and Haime, "Description des animaux fossiles du 

 groupe nummulitique de I'Inde," etc., livr. ii, p. 338, pl.xxxiv, figs. 12, 12^, 

 1854. The type-specimen, at one time in the Museum of the Geological 

 Society of London, and bearing the No. E. 9591, is now in the British 

 Museum. It is somewhat crushed, and consists of half of a whorl of the 

 septate portion of the shell, 73-5 mm. (1) in diameter. Its other measure- 

 ments are : height of outer whorl, 48 mm. (0-653) ; ditto above preceding 

 whorl, 26 mm. (0-353) ; greatest thickness, 44 mm. (0-598) ; centre of 

 siphuncle, 21-5 mm. from the ventral (peripheral) and 4-5 mm. from the 

 dorsal (inner) margin of the septum. The siphuncle is 4-5 mm. in diameter. 

 The umbilicus is obscured by matrix ; it was probably nearly closed. 

 There is a feeble umbilical shoulder just on the umbilical side of the saddle 

 on the lateral area. It is obvious from D'Archiac and Haime's fig. 12a, 

 which is fairly accurate (except that the outline of the umbilicus seems to 

 have been added), that the lower part of fig. 12 has been restored. 



^ J. Sowerby, Min. Conch., vol. i. No. 1 (June, 1812), p. 9, pi. i, upper, right- 

 hand (with septum in outline below), and middle figures; and J. de C. 

 Sowerby, op. cit., vol. vii, No. 109 (February, 1843), p. 35, pi. dcxxvii, 

 fig. 4. A completely septate example of this species in the British Museum 

 Collection from the London Clay of Primrose Hill, Middlesex [register 

 No. 50164] , has the following dimensions : diameter, 54-6 mm. (1) ; greatest 

 thickness, 38-8 mm. (0-71); height of outer whori, 34-1 mm. (0-624); 

 ditto above preceding whorl, 22-1 mm. (0-404) ; centre of siphuncle from 

 the dorsal (inner) edge of the septum, 9-4 mm. The measurements of the 

 present example of N. mokattamensis at a diameter of 56 mm. (1) are : 

 thickness of whorl, 42-0 mm. (0-75), and the height of the outer whorl, 

 31-6 mm. (0-564) ; and at a diameter of 64-2 mm. : thickness of whorl, 

 47-8 mm. (0-744) ; height of outer whori, 36-4 mm. (0-567) ; ditto above 

 preceding whorl, 23-3 mm. (0-362) ; centre of siphuncle from the dorsal 

 (inner) edge of the septum, 8-3 mm. 



^ E. Fourtau, Bull. Inst. Egypt., ser. IV, No. 1, p. 171, 1900 (1901). 



