118 L. F. Spath — Notes on Ammonites. 



a stable position along the angle bounding the venter". 1 Among 

 Pseudoceratites, e.g., Protengonoceras and Heterotissotia belong to this 

 group, and Hoplito ides in the young stage, though here the develop- 

 ment of an acute venter in the adult causes the disappearance of the 

 excentricity of the siphuncle. Solger saw an adaptation to a 

 benthonic existence in the latter, but the great variability shown 

 in the thirty-five specimens of Hoplitoides, quoted by Solger, points 

 to unstable conditions. This author also adduces the reduction of 

 the suture-line, the local restriction of the genus Hoplitoides, his 

 discovery of a specimen with several destroyed air-chambers, and 

 the similarity to the Triassic Ceratites, as evidence for the adaptation 

 to a benthonic existence. The writer has already given his opinion 

 on the first and third points, and the second does not now apply since 

 Hoplitoides has been found, e.g. in Tunis (Pervinquiere). "With 

 regard to the likeness of the Pseudoceratites of the Cretaceous to the 

 real Ceratites of the Trias, Phillipi's conclusions on which Solger's 

 theory of adaptation to a benthonic mode of life for both was based, 

 have also been disproved (Diener). 



Asymmetry in oxynote shells is very rare, and Swinnerton and 

 Trueman mention that they have not detected one case of asymmetry 

 in specimens with keeled venters (p. 54). The most notable example 

 is Gamieria heteropleura. Neumayr & Uhlig 2 examined about fifty 

 specimens of this and found the siphuncle excentric in each. It may 

 be assumed that the function of an oxynote venter was, primarily, to 

 assist rapid motion through the water, and only secondarily to act as 

 protection for the siphuncle, which in an unstable stock might vary 

 its position slightly, especially if the keel be hollow. Though this 

 last case is doubtful, however, the writer is inclined to think that 

 asymmetry of the siphuncle and suture-line cannot, by itself, be 

 taken as sound evidence in favour of adaptation to a benthonic, 

 crawling existence. 



In connexion with a pathological or accidental case of asymmetry 

 in a Perisphinctoid form, where the dorsal as well as the ventral 

 features of the suture-line are affected, Swinnerton & Trueman refer 

 again to the gas-pressure which, in Ammonites, was assumed to have 

 been strong enough to impose upon the septum a marked convexity. 

 They state (p. 52) : "If this [septum-secreting area of the mantle] 

 became hypertrophied on one side, it would still assume the form of 

 a stretched membrane, but being more resistant to pressure from 

 behind, would not become so concave forwards as the other half." 

 And on p. 37 the authors suggest that the second septum of Dactylio- 

 ceras already must have been formed under the influence of that 

 pressure, though at the time of formation of the protoconch and even 

 of the first septum, the mantle was strongly convex. 



Brief allusion to this pressure has already been made in connexion 

 with the phylogenetic " reduction " of the suture-line ; but in 

 Pseudoceratites, where the simplification is said to have been carried 

 to such extremes, the septum is still convex forwards. It seems to 



1 Op. cit., p. 55. 



2 ''Uber Amnion, a. d. Hilsbild. Nordd.": Palseontographica, vol. xxvii, 

 pp. 135-6, 1880-1. 



