﻿DUMORTIERIA RADIANS. 251 



together (" Polymorphidae," p. 138). The Levesquei-st&ge may be more or less 

 detected in figs. 11 and 12 of PI. XLII ; but in the other specimens it has been 

 superseded at an earlier age, and is therefore inconspicuous. 



The morphology of Dum. radians is given in PI. XLIIT, figs. 1 — 4. The 

 smooth stage suggesting Agassiceras miserabile and Polymorphites polymorphus is 

 clearly shown, and persists up to a diameter of 1^ lines. Then follows a ribbed 

 stage suggesting the change from the smooth Pol. polymorphus to the ribbed Pol. 

 polymorphus costatus, and parallel to the process which produced Am. Johnstoni 

 from a smooth ancestral form. As yet the ventral area is uncarinate; but later 

 on a keel is produced — observable at top of fig. 3, b. This may be said to com- 

 mence the Levesquei-st&ge, and is parallel to the change which produced carinate 

 species of Galoceras from uncarinate species. After the Levesquei-st&ge succeeds 

 the finer ribbing of "radians ;" and Plate XLIII, fig. 4, shows the process com- 

 plete. The gradual reduction of the gibbosity of the whorl from fig. 1 to fig. 4 

 should be noticed. 



From both Dum. Levesquei and Dum. striatulo-costata, Dum. radians differs in 

 its finer ribbing, and especially from the latter in the early age at which it com- 

 mences these fine ribs. From the fine-ribbed variety of Dum. striatulo-costata it 

 differs by its more open umbilicus and less occluded whorls ; and thus shows that 

 it is not a descendant of this form, but came directly from Dum. Levesquei. 



South of the Mendips Dumortieria radians is a rare fossil. I have specimens 

 from the Sand-rock of the Yeovil Sands {Jarense-zone, Dumortieria-beds) of 

 Bradford Abbas, Dorset, and have noted fragments at Ham Hill, Somerset, and 

 other places. From the so-called " Upper Lias " of Down Cliff, near Bridport, 

 Dorset — the blue clay which overlies the beds with Am. bifrons, communis, &c, and 

 underlies the Yeovil Sands — I have obtained undoubted examples. 1 



The Cotteswold specimens, which differ slightly from the south-country 

 examples, but differ much among themselves, are certainly more numerous. They 

 do not occur in the Cotteswold Sands, but in the overlying limestone capping. 

 Cam Down, near Dursley, and Penn Wood, near Stroud, are the principal 

 Gloucestershire localities ; Buckholt Wood and Sodbury have also yielded examples. 

 PI. XLT, figs. 4 — 8, illustrate two south-country specimens ; PI. XLII, figs. 1 — 12, 

 give the chief Cotteswold varieties of this species ; while PI. XLIIT, figs. 1 — 4, 

 exhibit the inner whorls of a specimen broken up in order to show development. 



1 Their presence in this clay shows that the clay is of much later date than would be supposed, and 

 is not equivalent to certain similarly-situated clays in other parts of England (see pp. 107, 168). It' 

 the Cotteswold Sands and Cephalopoda-bed be reckoned as belonging to the "Inferior Oolite 

 series," this clay must be reckoned there also. But see " On Cotteswold, &c, Sands," ' Quart. Journ. 

 Geol. Soc.,' vol. xlv, and "So-called Upper-Lias Clay of Down Cliffs," 'Quart. Journ. Geol. Soc.,' 

 vol. xlvii, pt. 3. 



