238 Dr. H. A. Nicholson on the Occurrence 
such an action, and an auxiliary, but decidedly feeble, power 
to the sete, there are many cases of molluscous excavation 
also in which chemical erosion has a large share in the result. 
3. A few words remain to be said as to the specific title and 
distinction of the lithodomous Leucodore. In the first place, 
it appears, from the remarks of Mr. Alexander Agassiz, fatel 
published in this Magazine, that Leucodore of Johnston is 
truly a synonym of Polydora of Bosc. Claparéde and De 
Quatrefages have both described species as belonging to the 
genus Leucodore, which do not possess the characteristic modi- 
fication of the fifth segment, and are really species of Nerine 
of Johnston. If this be so, Johnston’s name Lewucodore 
will have to fall. At the same time, it seems probable that 
both generic terms will be wanted; and I will not undertake 
to say how each shall be limited. The boring species does 
not differ obviously from Leucodore ciliata. I have not been 
able to make a comparison of specimens ; but it seems probable 
that they differ only in habit. Nevertheless, on account of 
this habit, it may be well to give Templeton’s title (which has 
precedence) of “‘ calcarea”’ to the boring form, which may be 
designated Polydora (Leucodore, Johnst.) calcarea. The same 
form of seta (fig. 10), the same copious ciliation of the branchial 
cirri which arch over the dorsum, the same remarkable anal 
cup or sucker (?), and the same form of head, with two tenta- 
cles which readily drop off, and a producible prostomium, 
characterize both P. ciliata and P. calcarea. It is quite pos- 
sible that more careful examination may disclose marked spe- 
cific differences, not only between these, but also between the 
various individuals boring limestone, chalk, &c. 
XXX.—On the Occurrence of the Genus Ptilograpsus in Bri- 
tain ; with Notes on the Ludlow Graptolites. By Henry 
ALLEYNE NicHoison, D.Sc., M.B., F.G.S. 
THE genus Ptilograpsus was originally described by Hall as 
occurring in the Quebec group in Canada; and two species 
have been differentiated by him, viz. P. plumosus and P. 
Geinitzianus (see ‘ Graptolites of the Quebec Group,’ pp. 189, 
140, pl. 21. figs. 1-8). In the Skiddaw Slates, our English 
equivalent of the Quebec group, no traces of this genus have 
hitherto been found; nor does it appear to occur in the Upper 
Llandeilo rocks of the south of Scotland, or in the Coniston 
Flags in the north of England—both especially rich in forms 
of Graptolites. Recently, however, by the kindness of Mr. 
Lightbody and Mr. Marston, of Ludlow, I have been furnished 
