DUMORTIERIA GRAMMOCEROIDES. 
265 
ancestral characters — that atavism is to be expected when the form is enfeebled 
by age or by injury. 
Dumortieria grammoceroides was named from a Dorset specimen in the Stras- 
burg Museum by my friend Dr. Haug in his excellent little pamphlet on the " Pohj- 
morphidae." I have to thank my friend for kindly sending me a plaster-cast of the 
original specimen, which shows that it is one of the somewhat involute forms, 
almost exactly like the specimen depicted in figs. 4, 5, of Plate XL VI, but with 
slightly coarser ribs. The name " grammoceroides " doubtless refers to the sub- 
sigmoid ribbing, which is more like what obtains among the species of the genus 
Grammoceras than is the usual " direct " ribbing of Dumortieria. 
The suture-line shows clearly that the present species is not a Grammoceras ; 
and, further, all the specimens lack the long ventral projection of the ribs peculiar 
to Grammoceras. 
For many years this species has been known to collectors of Dorset Inferior- 
Oolite fossils as one of the characteristic shells of the Goncavum-bedi. It has 
generally been named Levesqiiei,^^ "thouarsensis," or " striatulus" and is doubtless 
the species referred to by Dr. "Wright under the latter name as long ago as 1856.^ 
The involute specimens with subsigmoid ribs are easily distinguished from 
Bum. Levesquei ; but the evolute specimens are much like it. They have, however, 
coarser ribs, and in the inner whorls they have numerous closely-set ribs, essen- 
tially different from those of Dura. Levesquei. There is also a slight difference in 
the suture-line. It is worthy of notice that the adults are very similar in aspect 
to certain non-spinous species of Sonninia, but may be recognised by their 
suture-line. 
The descent of Dum. grammoceroides involves some amount of speculation, 
because it is rather distantly separated from its fellows. When broken up the 
inner whorls are too crystalline to afford much information ; although if they were 
otherwise they could not add much to what may be gathered from the outside of 
such an evolute form. At the first whorl the shell is smooth and uncarinate, 
involute, and gibbous-sided. The second whorl has coarse distant ribs like those 
of Dum. prisca, but the form is more involute generally than that species. I cannot 
say if there be any keel, but there is a faint one about a whorl later when the 
closer-set ribs are well established. Afterwards the ribs slowly increase 
proportionately in size. 
The second stage of ribbing seems to be too fine to have been inherited from 
"Levesquei;'' unless we imagine a phylogenetic series which started from 
"Levesquei" to attain finer ribs, and to force these ribs towards the centre by 
earlier inheritance, at the same time, assuming a coarser style of ribbing. 
These are the changes indicated by the specimens themselves ; but we know 
1 ' Quart. Journ. Geol. Soc.,' vol. xii, p. 292. 
34 
