116 L. F. Spath— Notes on Ammonites. 



asymmetry here is lost when the venter becomes acute. On the 

 other hand, when dissecting a specimen of, e.g., Psiloceras erugatum 

 (Bean-Phillips) with asymmetrical suture-line it is found that on the 

 innermost whorls the suture-line is quite normal and that the position 

 of the siphuncle (and of the ventral lobe) shifted gradually away 

 from the median plane, thus causing unequal development of the 

 opposing halves of the suture-line. Swinnerton & Trueman remark 

 that usually only the ventral features are affected, and enumerate 

 many genera in which this sporadical displacement has been noticed. 



These authors also point out that nearly every specimen in which 

 this type of asymmetry has been noticed has a rounded or flat venter. 

 The most notable example of the round-ventered shells is Psiloceras, 

 several signs of instability in which have already been noticed. 

 On a previous occasion 1 the writer attempted to demonstrate the 

 Monophyllites-Mojsvantes ancestry of this genus, and in this original 

 stock asymmetry apparently is unknown. In the Yorkshire repre- 

 sentative Psiloceras erugatum, a particularly variable species-group, 

 asymmetry may be associated with close or distant septation, or may 

 be absent altogether. Again, there may be approximation of the last 

 two to six septa or no approximation at all, with or without 

 "reduction". The specimens are also generally smaller than the 

 Somerset equivalent P. planorbis (Sowerby), which unfortunately is 

 always squashed, or the Wurtemberg P. psilonotum (Quenstedt), and 

 the Yorkshire examples remind one of the dwarf-forms of the 

 Hierlatz, so that one might be tempted to compare them with 

 organisms living under unfavourable conditions, such as the shells in 

 the brackish waters of the Baltic. But the enormous development of 

 the genus Psiloceras in the Alpine Hettangian, where asymmetry 

 is very common, shows that it was a dominant and thriving stock. 

 It may be assumed that there was a great "burst" when, with the 

 extension of the sea over wide new areas, the Ammonites which in 

 the Rhsetic had nearly become extinct, were able to spread again. 

 The one surviving family Phylloceratidse, reduced to a few secluded 

 localities of the Alpine PJisetic Sea, entered upon a new phase of 

 development, and in its principal first Liassic descendant Psiloceras 

 showed a strong adaptive radiation. 



It might be asked whether this could not have affected the position 

 of the siphuncle, and therefore the suture-line, in an attempt to 

 change a bilaterally symmetrical swimmer into a crawling benthonic 

 organism. For, as has already been pointed out, its probably 

 nectonic contemporary Phylloceras (which, like the somewhat later 

 " Rhacophyllites ", Euphyllites, Parapsiloceras, Pleuracanthites, and 

 Analytoceras, never migrated beyond the Alpine Hettangian Sea) 

 does not show these signs of instability. 2 Diener, who thinks the 



1 L. F. Spath, "On the Development of Tragophylloceras Loscombi" : 

 Q.J.G.S., vol. lxx, p. 352, 1914. 



2 Canavari (op. cit., 1882, p. 69) has thought that asymmetry of the 

 suture-line was not found in Phylloceratidse, but he figures as Amaltheus 

 (Sphenodiscus) sinister (ibid., pi. ii, xvi, fig. lla-c) a form of "Rhacophyllites" , 

 that clearly shows this asymmetry ; and both Pompeckj , and Swinnerton and 

 Trueman mention it as occurring in Tragophylloceras, but these are not typical 



