part 4] JURASSIC CHRONOLOGX: LIAS. 305 



MiCEODEROCERAS I^^FLATUM, Sp. nov. (PL XXVIII, fig. 3 ; 



PL XXIX, fig. 1.) 



Description . — Serpen ticone ; platy-perpaehygyral, perlatum- 

 bilicate to subpLaty-pachygyl'al, perlatumbilicate ; bituberculate, 

 septituberculate ; jDeripher}^ rounded, subarciHneate. 



Greneral details. — A much infiated development of Ammonites 

 hirchi, beginning with a similar smooth stage, rapidl}'^ develoj)ing 

 bituberculate, whereof presently the outer row becomes rather 

 stronger than the inner. Beneath spines and connecting rib there 

 is a septum of subspatulate shape. Close-set hair-like lines occupy 

 the interspaces and the arching periphery, which latter they cross 

 with a slight forward bend. 



Distinction. — The proportions (see below, p. 306) distinguish 

 this from preceding species : it attains at one time an infiation of 

 50 per cent, of the diameter, but afterwards declines as if com- 

 mencing to return to the hirchi style. 



Remarks. — According to indications of test on the periphery 

 the specimen had another complete whorl at least. Its diameter 

 may be roughly estimated, from the fact that from the centre to 

 the farthest periphery is 55 per cent, of the diameter of the last 

 whorl present : therefore the maximum radius becomes 45 per cent, 

 of the diameter of the next half -whorl, which being thus ascertained, 

 its maximum radius becomes "the basis, roughly 45 per cent, of the 

 second half- whorl. The indications of test on the periphery give 

 umbilical width of 54 and 62 per cent, of the respective diameters 

 (164 mm. and 198 mm.) so ascertained : these umbilical widths are 

 perhaps too great, because possibl}^ the 55 per cent, basis was not 

 maintained; and so the diameters are perhaps understated — the 

 whorl becoming thinner and broader. The umbilicus is expanding, 

 as the inner line is leaving the outer spine-line. 



As this specimen may be reckoned to have attained a diameter 

 of 200 mm. or more, it was, considering its inflated proportions, alto- 

 gether a much larger growth, had a greater bulk, than Ammonites 

 hirchi, though A. d'Orbigny does record a diameter of 240 mm. for 

 his specimen. The species now described possibly represented the 

 acme of development in Microderoceras ; but some of the later 

 Derocerates far exceeded these dimensions — some of the Radstock 

 giants collected by Mr. Tutcher and myself from the leckenhyi 

 bed attaining to 360-380 mm. (14-15 inches) in diameter. 



History of the figured specimen. — From hard light-blue 

 Lias weathered to light yellow and soft [white in places ?], from 

 excavations for reservoir of Bristol Waterworks, Barrow Grurney 

 (Somerset) — purchased. 



Microderoceras rhodanicum, nom. nov. 



Amnionites heherti Dumovtier {non Oppel), ' liassiu du Rhone ' vol. iii (1869) 

 pi. viii, tigs. 5-6 & p. 66. 



Remarks. — This name is for Dumortier's species quoted above, 

 Q- J. G.S. No. 292. 2 jj 



