Vol. 54.] DIVISIONS OP SO-CALLED ' JURASSIC ' TIME. 453 



There is a further reason in this. There is a family Arietidae, 

 and yet practically no genus Arietites. 



It seems probable that Sowerby's plate (' Min. Conch.' pi. cccclii) 

 contains species belonging to two genera. The upper figure is 

 now taken as the type of Turneri. 



Genus Oxyitotoceeas, Hyatt. 



The compressed species of the Arietidan Epoch, which are gene- 

 rally classed as Oxynotoceras, are, as their septal details show, 

 certainly poljgeuetic homoeomorphs — the terminal lines of dif- 

 ferent genera. Most of them are possibly extreme developments of 

 different genera of the Arietidae ; but there is also a possibility that 

 one or more species may be developments of Schlotheimia, and so 

 belong to the family Psiloceratidae. 



These matters can be settled only by long and careful working 

 out of ontogenetic and septal details. Mere outward form is of no 

 value in the matter of affinity. 



Genus Uptoi^ia, S. Buckm. 

 [Type : Uptonia ^ Jamesoni (J. de C. Sow.)'] 



The characters of this genus are that the spinous stage is never 

 strongly developed, but that it only takes the form of small knobs 

 upon the costse ; and that earlier or later, according to the degree of 

 development, these knobs disappear, giving place to a strong costate 

 stage. So a somewhat rotiform shape and strong, distant, straight 

 costae become the prominent features of the genus. 



Hitherto Jamesoni has been regarded as a Dumortieria^ but it and 

 its series are separable therefrom on account of a more ornate 

 septation. 



Genus Paltopledeoceras,^ S. Buckm. 

 [Type : Paltopleuroceras sjoinatum (Bruguiere).] 



This is merely a necessary alteration of name. Ammonites 

 spinatus and allied species have been hitherto known as PleurO' 

 ceras ; but that name is in prior use. The present description has 

 reference to the strongly-projected character of the costae on the 

 peripheral area — the way in which they appear to be thrown 

 forward. 



There are several species of this genus, but they have hitherto 

 been classed as ' spinatus and varieties.' The mode of growth, the 

 bisulcate periphery, and the simpler suture-line efiectually separate 

 the genus from Amcdiheus. 



^ In compliment to Mr. Charles Upton, who has explored the Jamesoni- 

 beds both in the South of England and in Scotland. 

 ^ TraXros, hurled. 



