238 Dr. H. A. Nicholson on the Occurrence 



such an action, and an auxiliary, but decidedly feeble, power 

 to the set£e, 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, lately 

 published in this Magazine, that Leucodore of Johnston is 

 truly a synonym of Polydora of Bosc. Clapar^de 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 Leucodore 

 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 dififer only in habit. Nevertheless, on account of 

 this habit, it may be well to give Templcton's title (which has 

 precedence) of " calcarea " to the boring form, which may be 

 designated PoJydora [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 ofi", 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 Oraptolites. By Henry 

 Alleyne Nicholson, 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. 2^lumosu^ and P. 

 Geinitzianus (see ' Graptolites of the Quebec Group,' pp. 139, 

 140, pi. 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 occm- 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 



