Vol. 54.] DIVISIONS OP SO-CALLED ' JFPwASSIC ' TIME. 445 



progenitors : the genera of the rotiformis and later hemerae are an 

 entirely distinct series, of different descent. This, I am aware, is an 

 extremely heterodox view, because Psiloceras planorhis is considered 

 as the radical whence the Jurassic ammonites have sprung,^ How- 

 ever, I read the affinities of this ammonite (planorUs) differently. 

 It is, in my opinion, a decadent form which has attained to the smooth 

 stage after its ancestors had passed through a ribbed stage, and of 

 this ribbed stage it shows traces itself in the obscure cost;© of its 

 inner whorls. Its ancestors, I consider, were slowly-coiled costate 

 species, like tortile or Johnstoni ^ ; and, in fact, these species are the 

 vigorous continuations of the same stock from which planorhis is a 

 decadent offshoot. With these species it agrees in another character, 

 namely, that the inner lobes are dependent, and point in an oblique 

 manner across the whorl towards the periphery ; bnt this is just the 

 character which is not found in the Arietidae, as I should define them, 

 and on the absence of this character in the Arietidae dominating the 

 Asteroceratan Age I rely to show that they are not the descendants 

 of Psiloceras. 



Another view of the descent of Psiloceras planorhis has been taken 

 by Mojsisovics : he considers it to be the descendant of Monophyllites 

 Clio, a Triassic species from what he calls the ' Juvavische Stufe.' 

 Although this might rather favour the chronological arrangement 

 for which I argue, yet there are, to my miad, insuperable difficulties 

 against its acceptance. The weightiest of these is that Mono- 

 phyllites Clio does not, according to Mojsisovics's own figures, show 

 that dependent character of the inner portion of the suture-line 

 which obtains in Psiloceras planorhis. How then is it that Psiloceras 

 planorhis shows this character, which it shares with other species ? 

 But there is a further objection : Monophyllites Clio does not show 

 those ribs which should be found in a ^Sirent of Psiloceras planorhis, 

 according to the evidence of the inner whorls of the latter species. 



There still remains a large series of species in the ])ve-roliformis 

 hemerae, which have been called Arietites or Arietidae, and this might 

 be thought to tell against my view ; but in connexion with these 

 species certain essential details have been disregarded. Thus Hyatt 

 places in his genus Caloceras not only species such as liassicum and 

 tortile (which have complex suture-lines with the inner portions 

 strongly dependent, and the inner lobes pointing obliquely, across the 

 whorl towards the periphery), but also species such as raricostatum, 

 which have a simple suture-line running straight across the whorl 

 and not dependent. Here there are species not only of two genera, 

 but really of two families. Caloceras must be restricted to the 

 first ; Bayle's Echioceras is used for the second — the raricostatum 



^ ' Psiloceras planorhis is a radical derived from Ps. caliphyllum or else from 

 pre-existing Triassic ancestors, and the absence of a complete series connecting 

 it or Ps. caliphylium with Gymnites of the Trias is evidently due to the absence 

 of an equally complete series of formations,' Hyatt, ' Genesis of the Arietidge,' 

 Smiths, Contrib. Knowledge, vol. xxvi (1889) No. 673, p. 117. 



^ Compare possibly Ammonites IcBvidorsatus, von Hauer, ' Nachtr. zur Kenntn. 

 der Ceph. Fauna d. Hallstatfcer Schichten,' Sitzungsber, k. Akad. Wissensch. 

 Wien, vol. xli (1860) pi. iii, figs. 9 & 10. 



