Nov. OPPELIDM 25 



7920 



a thinner and smaller form than Brasil's species. His dimensions are, 

 from beginning of depression before mouth through umbilicus, T. & F. 15.5 

 (31), 58, 48, — , and this one is much the same at a diameter of 10.75, 

 taken at the same place — consequently, at this diameter Brasil's species 

 is much thicker (see preceding species). 



The prorsiradiate primary ribs bifurcate into decreasingly rursi- 

 radiate secondaries. Just before the angle the primaries nearly disappear 

 and the secondaries pass rather suddenly into fine ribs. 



Broad Windsor, Dorset — the suggested date, truellei hemera, would 

 accord with that of the Burton-Bradstock species.' 



Family OPPELIDJE, S. Buckman 1898. 



Oppelia, Waagen, 1869, (Form. Am. subradialus ; Geogn.-Pal. 

 Beitr. 11.(2), 250). Genosyntypes, O. subradiata s.l. ; Genolectotype, 

 0. subradiata, Waagen, Id. PL xvi, f. 1, (non Am. subradiatus, J. de C. 

 Sowerby) = 0. waageni nom. nov. This form shows major ribs at 

 intervals and short minor ribs in the interspaces : these die away, leaving 

 major ribs only, perhaps relics of the bullate ribs of Diplesioceras, see 

 below. 



Oecotraustes, Waagen, 1869, Id. p. 251 (Oekotraustes). Geno- 

 holotype, Oecotraustes geniculates, Waagen, Id. PI. xx, f. 4a, b. Ribs 

 are uniform in this genus. 



Diplesioceras, g.n. (Sin-X^o-tor, twice as much). Genoholo- 

 type, D. diplesium n. PI. CLXXVII. Serpenticone ; carinate- 

 bisulcate ; costate, with, at intervals, a double-sized rib ending in a bulla 

 on latero-peripheral border. This genus is remarkably like Dipoloceras, 

 Hyatt, of the Lower Cretaceous, but the strong rib does not fork, instead 

 it remains single, and swells into a strong bulla or blunt transverse knob. 



This genus presumably belongs to the family Oppelidae, and shows 

 in its intermittently strong costation a phase through which Oppelia 

 and like forms have presumably passed; for there remain in such forms 

 short ribs at intervals which would correspond to the bulla; and be 

 relics of their former existence — the intermediate ribs and other part of 

 the strong ribs having disappeared in catagenesis from serpenticone to 

 oxycone, retained latest around the keel. If this supposition be correct, 

 then Oppelia-\ike forms have had a much more varied history than has 

 been supposed — they have rapidly dropped the serpenticone, carinate- 

 bisulcate, tuberculate stage, and have become examples of quick trans- 

 formation with skipping of stages, pronounced lipopalingenesis. 



England (Dorset). Vesulian (Eningenian), garantiana. 



D. diplesium nov. PI. CLXXVII, sufficiently described in 

 generic diagnosis. Vetney Cross, near Bridport, Dorset ; S. 15, 40, 

 53 (4°). 28 ; S.B. Coll. No. 3265, Genotype and Holotype. 



Family PERISPHINCTIDM, Hyatt, 1900. 



This was proposed for the very large series of costate serpenticones 

 which characterize the Middle and Upper Jurassic strata ; but they 

 are of polyphyletic origin. The family diagnosis must be deferred until 

 the genotype of the typical genus Perisphinctes has been selected ; for 



