﻿DUMORTIERIA RADIANS. 249 



Of the specimens which I have had delineated none agree absolutely with 

 Reinecke's figure, because they do not possess so strong a carina (p. 186). The 

 specimen depicted in PI. XLI, figs. 7, 8, seems to me to agree in every other 

 respect. This specimen is from the south of the Mendips ; l and the fossil exhibited 

 in the same plate by figs. 4, 5, which is also from the same district, and which agrees 

 exactly with Dumortier's figs. 3, 4, of "Am. radiosus," is either a larger example 

 of the same form, or is very closely related. The Cotteswold specimens differ 

 slightly from these south-country examples, because they have generally a more 

 compressed whorl, and a slightly sharper ventral area (figs. 2, 9, 10, PI. XLII, 

 are exceptions), but notably because their whorls are proportionately broader, and 

 are not so numerous as in Reinecke's figure — in fact, the Cotteswold specimens 

 are a trifle quicker-coiled. It is in this matter of coiling that the specimen de- 

 picted in PI. XLI, figs. 7, 8, is so noticeable. 



In the matter of compressed whorl the specimen figured in PI. XLII, figs. 6, 

 7, goes furthest; and what with this, and its more occluded whorls and finer 

 ribbing, it differs from the typical Bum. radians, and is, in fact, a link connecting 

 this species with Dum. Moorei. 



All the specimens agree with Reinecke's figure in one important respect, 

 namely, in the " direct " ribbing. This shows that it is a mistake to identify 

 Reinecke's figure with any species of Grammoceras. 



If the species of Dumortieria be compared with the series of specimens of the 

 genus Grammoceras illustrated in this Monograph, it will be seen that the 

 manner in which the ribs cross the lateral area distinguishes the species of the 

 two genera throughout. This direct rib of Dumortieria is, however, more im- 

 portant in another way, because it is the outward index, as it were, of a different 

 suture-line — a fact most noticeable, however, in the less-developed species of 

 Dumortieria (PI. XXXVII). Further, the direct rib is an indication of a different 

 stage of development, for the direct rib is common to the ancestors of Dumor- 

 tieria and of Grammoceras ; but Grammoceras itself, which has passed through 

 more changes than Dumortieria, has acquired a more sigmoidal style of ribbing 

 (pp. 159 et seq.). 



Ammonites radians is a species which has been quoted with very great fre- 

 quency ; and, owing possibly to the poorness of Reinecke's figure, the specific name 

 "radians" has been applied by different authors to a large number of species 

 ranging from the Middle Lias to the Inferior Oolite. The synopsis given at 

 pp. 188 — 190 is an attempt to grapple with the subject ; and it shows which 

 of the various figures of Ammonites called radians belong to the species as now 



1 The Dorset-Somerset basin formed part of the Paris basin, and was cut off from the 

 Cotteswold area by an extension of the Mendip axis. See ' Proc. Cotteswold Club,' vol. ix, pt. iv, 

 pp. 374—387. 



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