THE CAMBRIDGE GAULT AND GREENSAND. 287 



cimens ; that named Ammonites Wiestli in the British Museum, and 

 described by Mr. Seeley, belongs to the same species. 



Ammonites planulatus, Sow. 



Ammonites planulatus, Sow. Min. Conch, pi. 570. f. 5 ; Sharpe, 

 ChalkMoll.pl. xii. f. 4. 



Ammonites Mayorianus, D'Orb. Pal. Fr. i. p. 267, pi. lxxix. ; 

 Pictet, Gres Yerts, p. 37, pi. ii. f. 5. 



Ammonites octosulcatus, Sharpe, pi. xix. f. 3. 



Ammonites Griffithsii, Sharpe, pi. xi. f. 3. 



These appear to be only varieties of the same form (see the re- 

 marks of Mr. Seeley in Ann. & Mag. Nat. Hist. ser. iii. vol. xvi.). 

 A. planulatus was originally described from a Chalk-marl specimen, 

 and may be retained for the thicker forms of the group with a wider 

 umbilicus, while the flatter forms so common at Cambridge agree 

 better with those called A. Mayorianus. 



It is interesting to find that these varieties succeed one another in 

 point of time ; MM. Pictet and Campiche (Ste.-Croix, p. 285) re- 

 mark that the flatter types approach nearer to A. Emerici, an 

 Aptien form, and that they are chiefly found in the Gault inferieur, 

 which immediately succeeds the Aptien, " while A. planulatus 

 becomes more frequent in the upper beds of the Gault and in the 

 beginning of the Cenomanien period." 



The relative abundance and state of preservation of the Cam- 

 bridge specimens quite agree with the above remarks. The variety 

 Mayorianus is always in black phosphate, and has come from beds 

 some way down in the Gault ; while the much rarer planulatus is 

 preserved in brown phosphate, and has probably been derived from 

 the upper sandy beds like those between the Gault and Chalk-marl 

 described in the first part of this paper. 



Many such examples of development are only waiting to be de- 

 scribed by palaeontologists, and to form strong evidence for Mr. 

 Darwin's theory. 



Scaphites Meriani, Pict. & Camp. PI. XIV. figs. 1-3. 



Scaphites Meriani, Pict. & Camp. Ste.-Croix, ii. p. 16, pi. xliv. 



?v&. Hugarclianus, D'Orb. ; Pictet, Gr. Yerts, pi. 12. f. 2. 



The Cambridge Scaphites have always been regarded as S. cequalis ; 

 but Mr. Seeley correctly describes the form in Ann. & Mag. Nat. 

 Hist. ser. 3, vol. xvi., and notices that there are three varieties : one 

 he describes as the type which is ornamented with simple ribs, 

 slightly elevated at the sides, and bifurcating before crossing the 

 back : another he mentions as possessing a row of tubercles on each 

 side ; this form agrees with the S. Meriani, figured and described 

 by MM. Pictet & Campiche. M. Renevier, however, marks S. 

 Hugarclianus as occurring at Cambridge (Faune de Cheville, p. 111). 

 This species, as figured in the ' Gres Yerts,' has only three tubercles, 

 and the ribs are never obliquely curved, but run straight over the 

 shell. I very much doubt whether this is more than a variety in- 



