286 A. J. JUKES-BROWNE ON THE RELATIONS OF 



Ammonites rostratus, Sow. 



Ammonites rostratus, Sow. Min. Conch, pi. 173; Ooster, Ceph. 

 Alp. Suiss. p. 142, pi. xxvi. f. 1-3. 



Ammonites inflatus, Sow. Min. Conch, pi. 178; Pict. & Campiche, 

 Ste.-Croix, p. 178, pi. xxi. f. 5, pi. xxii. f. 3, 4. 



Ammonites symmetricus, Sow. Geol. Trans. 2nd ser. iv. pi. xi. f. 21. 



The form found in the Gault and Upper Greensand of England, 

 which is large, somewhat compressed, and has fewer ribs, seems 

 to he Sowerby's original A. rostratus ; and the smaller, compressed, 

 strongly ribbed form so common at Cambridge and also at Cheville 

 is closely allied to this. They undoubtedly pass, however, into the 

 A. inflatus , Sow., as figured by D'Orbigny and Pictet and Campiche, 

 assuming an inflated shape, with numerous ribs broken up into 

 tubercles ; to this the inflated and closely ribbed form called by Mr. 

 Seeley A. pachys is very nearly allied, and hardly constitutes a 

 third variety*. A. rostratus being so abundant in the Upper Gault, 

 and not being found below, may well give its name to the horizon, 

 and consequently to the Cambridge nodule-bed, in which it equally 

 abounds. 



Ammonites dispar, D'Orb. 



Ammonites dispar, D'Orb. Pal. Fr. p. 143, pi. 45. f. 1, 2 ; Pict. 

 & Camp. Ste.-Croix, p. 264, pi. xxxviii. 



In his excellent paper on the Ammonites of the Cambridge Green- 

 sand (Ann. & Mag. Nat. Hist. ser. 3, vol. xvi.) Mr. Seeley describes 

 some Ammonites under the name of A. navicularis, var. nothus, 

 correctly noticing that " they are more flattened than is usual in 

 examples of A. navicularis from the Chalk, and differ in never having 

 any tubercles ; the umbilicus is also commonly smaller." 



In form they certainly approach A. Mantelli, and I have seen 

 specimens of this species in which the ribs passed over the back 

 without developing tubercles. The smaller Cambridge specimens 

 might readily be taken for A. Mantelli, and, indeed, Mr. Etheridge 

 was inclined to place the one I showed him with that species. 



There are, however, two specimens now in the Woodwardian 

 Museum of more adult age, in which the ribs become more distant, 

 and finally more faint, apparently passing through some of the 

 stages depicted in pi. xxxviii. of Pictet and Campiche as those of A. 

 dispar. At p. 267 they remark : — " When this species is complete, 

 and one can follow its modifications, it cannot be confounded with 

 any other. When young it rather resembles A. Mantelli and A. 

 Milletianus ; it is always distinguished, however, by its more rapid 

 * enroulement, ' its narrower umbilicus, and the disposition of its 

 ribs, which, instead of alternating regularly large and small, are of 

 unequal number, the larger being much less abundant and separated 

 by several small ones." These characters equally apply to our spe- 



* It appears to me identical with that figured by MM. Pictet and Roux, Gres 

 Verts, pi, 9. f. 6. 



