﻿DUMORTIERIA RADIOSA. 253 



Dumortiehia eadiosa (Seebach). Plate XLII, figs. 13 — 15; Plate XLIH, figs. 5—7. 



Woodcut in the text, fig. 3, p. 187. 1 



1864. Ammonites badiosus, Seebach. Hannoveriscbe Jura, pi. ix, figs. 2« — c. 

 1875. — Moobei, Lepsius. Beit. Kennt. Juraf. Unter-Elsass, pi. ii, fig. 6. 



1885. Hahpocebas eauiosum, Haug. Beitr. Monogr. Harpoeeras ; Neues Jahr- 



buch fur Mineral., &<•., Beil.-Bd. iii, 

 p. 665. 

 1887. Dumoetieeia badiosa, Raug. Polymorphidse ; Neues Jahrbuch, &c, Bd. 



ii, p. 140. 



Discoidal, compressed, carinate. Whorls ornamented, first, with coarse distant 

 ribs, later with very fine growth-lines. Ventral area with small, inconspicuous 

 carina. No inner margin. Inclusion about one-third. Umbilicus with somewhat 

 tumid whorls. 



Dr. Haug 2 writes to me that he entirely approves my identification of the 

 specimen depicted in PI. XLIII, figs. 5, 6 ; but I am somewhat inclined to think 

 that it is not only a trifle more compressed than Seebach's figure, but that its 

 umbilicus is too large. 



The differences between this species and the Cotteswold forms of " radians " 

 are very small, and I was at one time inclined to treat radiosus as a synonym 

 thereof. The whorls are, however, just a trifle broader, and the specimen is 

 slightly quicker-coiled — in fact, in this matter they differ from the Cotteswold 

 forms of radians about as much as the latter differ from the south-country forms. 

 Possibly the best distinction is the coarse ribs in the inner whorls changing so 

 suddenly to the fine strise ; because this feature probably indicates that Dum. 

 radiosa is a direct mutation of Dum. Levesquei, and is not a variety, but is a 

 " cousin " of Dum. radians. The Levesquei-stage may be clearly seen in PI. XLII, 

 figs. 13, 14. 



Dum. radiosa is a rare species ; and I have only met with it at Cam Down, near 

 Dursley, and Penn Wood, near Stroud. It came from the Moorei-heds of the 

 latter place ; but I am not certain that it was not found in the Dumortieria-beds 

 of the former locality. These two horizons — the top of the Jurense-zone 

 and the bottom of the Opalinum-zone — are very intimately united in several 

 places. 



PI. XLII, figs. 13, 14, illustrate a small specimen showing the sudden change 



1 This specimen is probably more typical of Dum. radiosa than the British examples depicted in 

 the plates. 



2 I take this opportunity to thank my friend for his kind and critical remarks upon my proof 

 plates. These remarks have been of the greatest assistance to me in checking my determinations. 



