L. F. Spath — Notes on Ammonites. 115 



systems of Megacystis, each with its single diplopore, do not seem 

 like direct modifications of the Aristocystis plan. It is curious that 

 this type of channel-system should, to all appearance, have heen 

 confined to a single arm of a single sea during one relatively brief 

 period of the earth's history. 



EXPLANATION OF PLATE III. 

 Porks and Channel- Systems in Megacystis. 

 Fig. 14. — Openings of two diplopores on the inside of the theca in E 7671. 



15, Three diagrams to show the multiplication of the channels from 



(a) the primitive two, through (b) four, to (c) five. 

 ,, 16. — Simple channel-systems, corresponding to stages a and b of Fig. 15, 



seen in E 7676. 

 ,, 17. — A portion of the worn thecal surface in E 7639, showing more com- 

 plicated channel-systems. In a the two pores are distinct, but 

 there is a deceptive appearance of a pore where the short channel 

 ends on a bar. In b the pore to the right is divided by a bar. 

 In c both pores are so divided. 

 ,, 18. — A portion of the unworn thecal surface of E 7633. Two channel- 

 systems are faintly seen beneath the epistereom. 

 ,, 19.— A portion of the unworn thecal surface of E 7673, showing pustules. 

 Here conditions of petrifaction are such that the underlying 

 channels can nowhere be detected. 

 „ 20.— Pustules in various stages of weathering; from the deceptive 

 appearance of many pores, through the exposure of the channel- 

 system, to the beginning of its disappearance. Selected from 

 E 7629. 

 ,, 21.— Other stages in the wearing down of the pustules till the primitive 

 diplopore is exposed. Selected from a single plate in the theca 

 of E 7672. 

 The figure-numbers continue those of the text (Nov. and Dec, 1918). 

 All figures enlarged 20 diameters. 



IV. — Notes on Ammonites. 



By L. F. Spath, B.Sc, F.G.S. 



III. 



IN addition to the variability of the suture-line in a given species, 

 mentioned previously, asymmetry of the elements on opposite 

 sides of the same suture-line is very frequent and probably universal 

 in so far as the minor frillings are concerned, which is only to be 

 expected in organic beings. This phenomenon has lately been 

 illustrated again in Lioeeras by Horn, 1 and in Dactylioceras by 

 Swinnerton & Trueman. 2 The latter authors also have some 

 interesting observations on asymmetry associated with lateral 

 displacement of the siphuncle which is of sporadical occurrence in 

 Ammonites. 



Canavari 3 had noticed that in some Spezia forms the siphuncle 

 was asymmetrical in the young- and then became central; and 

 Solger i records a familiar excentricity in Hoplitoides. The 



1 " Die Harpoceraten der Murchisonse-Schichten d. Donau-Rhein Zuges " : 

 Mitt. Grossherz. Bad. Geol. Land. Anst., vol. vi, pt. i, p. 264. 



2 Op. cit., Q.J.G.S., vol. lxxiii, pt. i, pp. 40, 51, 1917. 



3 " Beitr. z. Fauna d. Unt. Lias v. Spezia " : Palffiontographica, vol. xxix, 

 pt. iii, p. 192, 1882. 



* " Fossil d. Mungo-Kreide " : Geol. v. Kamerun, ii, p. 217, 1904. 



