'292 DR. L. F. SPA.TH ON [vol. lxxix, 



drawn to the difficulty of distinguishing these Paraclasyceras 

 "from their contemporaries Phylloceras and G-eyeroceras. 1 



I noAv consider it probable that from this persistent radical 

 stock of e volute Phylloceratids, including, in the Middle and 

 Upper Jurassic, the genus Sowerhyceras, have successively ori- 

 ginated tbe ' cryptogenous ' families of trachyostracous ammo- 

 nites, some by way of transitional types of the Lissoceras, Haplo- 

 ■ceras, and Desmoceras pattern, erroneously united into ' families.' 

 The genus now denned as Rhacopliyllites (but not its specialized 

 adults) thus probably gave rise to Tragophylloceras," and the 

 genus Amphiceras and the family Amaltheida? may have sprung 

 from the same radical Plrvlloceratid stock. 



The modified offshoots with robust ornamentation generally 

 show simplification of the suture-lines, changes that are probably 

 effected in the early stages and correlated with adaptation to a 

 mode of life different from that of the leiostracous parent stock 

 which never left the warmer seas of the Tethys. 



Other modified Phylloceratids, such as the 'Purbeckian' Phyllo- 

 ceras strigile (Blanford), the ornamentation of which somewhat 

 resembles that of the contemporaneous, dominant ammonite family 

 Berriasellidag, leave no progeny, and remind one of the curious 

 simultaneous appearance of the umbilical processes in the Lower 

 Triassic Otoceras and its contemporary Nautili, perhaps examples 

 •of mimicry. 



I^hacophtllites aff. Diopsis (Gremmellaro). (PI. XVIII, 

 figs. 2 a--2 c.) 



1884. Phylloceras diop>sis Gemmellaro, ' Sui Fossili dei Strati a Terebratula 

 aspasia, &c.' pt. i, Soc. Sci. Nat. & Econ. Palermo, p. 6 & pi. ii, 

 figs. 6-8. 



The specimen here described consists of the body-chamber 

 portion of about half a whorl, of an ammonite some 70 mm. in 

 diameter, but showing signs of crushing. The umbilicus is nearly 

 '25 per cent, of the diameter, or about the same width as in the 

 Sicilian type. The resemblance of the ornamentation to that of 

 certain Tragophylloceras, notably T. ibex-heterophyllum (Quen- 

 stedt), 3 is partly due to the crushing of the whorl and its 

 periphery. There is a likeness to a Rhacopliyllites of the gigns 

 type from the Toarcian (?) of the Kammerker Alpe, Tyrol (Klip- 

 stein Coll. B.M. No. C 13856), but less resemblance to the Lower 

 Liassic Campiglia forms (Rhacopliyllites narclii Meneghini sp.) 

 in the British Museum collections. 



The beginning of the body-chamber shows portions of the 

 saddles of the last suture-line, but not the lobes. The external, 

 'first and second lateral saddles are very distinctly diphyllic, the 



1 L. F. Spath, Q. J. G. S. vol. lxx (1914) pp. 354-55. 



2 'Notes on Ammonites' Geol. Mag. vol. lvi (1919) p. 29, footnote 4 & 

 p. 221. The 'recapitulation theory' has since been practically abandoned. 



3 ' Ammoniten des Schwabischen Jura ' 1885, pi. xxxvii, fig. 14 & p. 294 = 

 Phylloceras sp. in Pompeckj, op. cit. 1893, pt. i, pp. 13 & 18. 



