DORSETENSIA. 
303 
greatest break is between Dorsetensia Edouardiana and Dors, complanafa, n. sp. ; 
and it separates the members of the genus into two groups. (Page 312.) 
It is a question whether Dors. Edouardiana is a descendant of Ha.plopleuro- 
ceras subspinatum, which has lost the spines of that species by earher inheritance 
of a retrogressive costate stage. In general whorl-shape and in ribbing the two 
species are similar, while both possess the carinate-sulcate ventral area ; but 
whether Dors. Edouardiana would, if descended from ^'^ subspinatum have lost 
the spines so completely, and yet have retained such well-marked ribs, is a matter 
on which I feel considerable doubt. I certainly placed " Edouardiana " and its 
allies in their present position in the plates with the idea that they might be 
descended from " subspinatum ;" but I confess that I am not able to settle the 
matter definitely. 
The inner whorls of Dors. Edouardiana disclose a globose, gibbous- whorled form 
with a coronet of spines (PI. XLIX, fig. 4), and these spines gradually develop 
into ribs (PI. LII, fig. 14), which are retained throughout life. The spinous inner 
whorls point to descent from the common ancestor of the Sonnininae ; but, if 
" Edouardiana " is not derived from " subspinatum,''^ it may be supposed that its 
ancestor was a form which only attained the unispinous stage, and that Dorsetensia 
exhibits the various phases of retrogression therefrom to costate and striate 
stages. I think this idea is more probably correct, and this might bring 
Dorsetensia in closer relationship with Witchellia. 
From Haploplewoceras, Dorsetensia Edouardiana difi'ers only in not possessing 
the two rows of spines ; but the absence thereof is, I think, sufficient reason for 
placing it in a separate genus in any case, especially as the other species of the 
genus difier materially from Haplopleuroceras. 
From Witchellia it differs in wanting, practically speaking, any spines or 
knobbed ribs, and in not having a hollow carina until the species h^a^ become 
smooth. In Witchellia the hollow carina is correlated with evolute whorls, strong 
ribs, and ventral sulci ; while these ventral sulci are retained on the core when 
the species are nearly smooth. This is not the case with Dorsetensia, which lost 
the ventral sulci long before the hollow carina made its appearance. 
Although the species of Dorsetensia are said to retrogress in the matter of 
ornament, yet they progress so far as actual size is concerned. The species of 
the genus make a complete change from evolute, well-ribbed, sulcate-carinate 
whorls, to involute, smooth, hollo w-carinate whorls. 
All the species of the genus are found in the Humphriesianum-mne with the 
exception of two forms, of which the material is very scanty — not sufficient for 
exact determination. These two forms occur in the 8auzei-zone. 
