L. F. Spath — Notes on Ammonites. 69 



approximation, then wide intervals, and approximation again of the 

 last two septa. 



Amioceras nodulosum (J. Buckman) presents a similar variahility, 

 to judge by nineteen specimens examined. Among twenty-one 

 specimens of Peltoceras aff. Eagenii (Raspail) there were seven that 

 showed no approximation at all, and the majority of the remainder 

 had only the last septum slightly closer. In only two specimens 

 they were very close, and only one had the last three septa gradually 

 approximating. In three specimens the distance between the third 

 and second septum from the end was greater than that between 

 either the fourth and third or between the second and the last. It 

 was also noticed that in two specimens the last suture-lines (in both 

 cases only slightly approximating) were much fainter than the 

 others, indicating a thinner septum instead of a thickening as in the 

 case of the Nautili mentioned, or else the beginning of deposition of 

 calcium-salts on a conchiolin-membrane as noticed by Swinnerton & 

 Trueman. 



Again, R. Douville ' found a great variability in the spacing of the 

 septa in Quemtedticeras lamberti, and he figures two specimens which 

 are very different in this respect. Douville adds: "As I have 

 observed that in many dwarfed and cecotraustic forms, (which are in 

 all probability the males, Chapuisi, etc.) the septa are always 

 extremely close, it seems to me that one could see a certain connexion 

 between small size and close septa. The Quenstedticeras with close 

 septa would never have obtained great dimensions, and would be 

 the males, and vice versa. Unfortunately there are too many 

 transitions among the various distances of the septa to enable one to 

 think of demonstrating this hypothesis. But it also does not 

 disprove the theory, for if the spacing of the septa be a secondary 

 sexual character in Quenstedticeras. it is possible that it may be very 

 unequally developed in {he individuals of the two sexes." 



It seems to the writer that with regard to sexual dimorphism in 

 Ammonites, the evidence to-day is as unsatisfactory as it was when 

 Buckman and Bather inquired into this problem, and came to the ■ 

 conclusion that "sexual dimorphism had yet to be proved". 2 The 

 two Ammonites figured by Douville differ, however, e.g., in 

 the width of the umbilicus, and in the writer's specimens of 

 Amioceras the suture-lines were by no means all alike, so that it 

 might be suggested that they represent several types, perhaps due 

 to hybridization of the extreme members. As all my specimens of 

 Amioceras niger came out of one block it cannot be a question of 

 different surroundings. Slight differences in the character and 

 thickness of the shell or of the septa, and the simplification of the 

 edges of the latter, might account for the variability in individuals 

 of the same species ; but rate and arrest of growth would differ 

 especially if either the stock itself or the environment were unstable. 

 What Edwards 3 said with regard to the London Clay Nautili may be 



1 Op. cit. (Cardioeeratides), p. 61, fig. 60. 



2 "Can the Sexes in Ammonites be distinguished?": Nat. Sci., vol. iv, 

 p. 430, June, 1894. 



3 The Eocene Mollusca, pt. i : Cephalopoda (Pal. Soc. Mom), 1849, p. 44. 



