10 THIRD SUPPLEMENT TO THE CRAG MOLLUSC A. 



In the ' British Mollusca," and in the ' British Conchology/ there are more than 

 twenty Odostomia described as distinct species, each with very slight differences of 

 character ; but whether they are all specifically distinct is perhaps questionable. The 

 Authors of ' British Mollusca,' vol. iii, p. 260, justly say : " The species are difficult to 

 distinguish and very critical." I have figured several so-called species under this generic 

 name and I have in most cases assigned them from the figures and descriptions of these 

 Authors, and of the Author of ' British Conchology/ as they had better means for 

 determination than I have had. 



[The specimen figured is probably one which has been carried into the fluvio-marine 

 Crag from the same bed as that which supplied Cerithium derivatum and Odostomia 

 derivata. ED.] . 



[The following description of a new species and some remarks as to the bed at 

 Boy ton, in which it occurred, have been kindly supplied by Mr. Robert Bell. ED.] 



[MARGARITA CRASSI-STRIATA, Eobt. Sett. 3rd Sup., Tab. 1, fig. 15. 



Locality. Boyton. 



Shell small, very solid, somewhat conical ; whorls Jive ; suture deep, each volution 

 having four or Jive thick revolving ridges with traces of Jine intermediate ridges. These 

 are crossed by prominent lines of groioth, giving them a slightly crenulated appearance. 

 The base is, like the whorls, rounded and strongly ridged, with a very small umbilicus. 

 Mouth rounded, with an obscure tooth or fold near the base of the columettar lip. 



The species which seems nearest to it is Margarita cinerea, Couthuoy, but it differs 

 in having much stronger ridges, especially at the base, and a smaller umbilicus. The 

 upper whorls also do not seem to have that lattice-like appearance which is present in 

 well-preserved specimens of M. cinerea. 



It is difficult to indicate which formation this shell belongs to. The section of Crag 

 worked at Boyton can seldom be seen, being an excavation close to the Butley River, and 

 mostly from three to six feet under water, the coprolite diggers standing in the water 

 when at work, and scooping up the sand from the bottom of the trench ; but from what 

 I have been able to observe, and from an examination of a large number of species 

 found there, the formation seems to range from the fossiliferous beds of the Coralline 

 (Zone d. of Prestwich's section in his paper on the " Crag Beds of Suffolk and Norfolk," 

 ' Quart. Journ. Geol. Soc.,' vol. xxvii, p. 121,) up to the middle portion of the Red Crag. 

 Probably some of the beds have been reconstructed from the wearing away of the Upper 

 Coralline strata on the other side of the river, although a bed of the larger bivalves 



