378 Mr. N. T. Wetherell on a Barytic Deposit 



EXPLANATION OF PLATE XV. 



Fig. 1. General view of the interior of Pholas crispata, the mantle being 

 laid open along the ventral margin : — a, incurrent siphonal orifice, 

 with a piece of whalebone passed through it to show the direction 

 of the current ; b, excurrent siphonal orifice, with a piece of whale- 

 bone likewise passed through it into the anal chamber ; c, the gills, 

 exhibiting the indigo collecting on their surfaces in minute streams 

 and passing along their free margins ; d, anal chamber laid open, 

 exhibiting the four rows of orifices leading into the tubes between 

 the gill-laminae ; e, e, e, labial tentacles ; f, accumulation of in- 

 digo on them, brought by the gills ; g, oral orifice ; h, foot. 



Fig. 2. Section of the gill-plate, showing three of the tubes between the 

 laminae : — a, the tubes. 



Fig. 3. A portion of the gill-laminae highly magnified, showing the orifices 

 in the meshes. 



XXXI. — Notice of a Barytic Deposit in certain Testacea from the 

 London Clay, By N. T. Wetherell, Esq., F.G.S., M.R.C.S. 



During the examination of the Testacea of the London clay, I 

 have observed in some univalves from a particular locality the 

 occurrence of small concretionary bodies frequently filling the 

 apertures and umbilici of the shells. These bodies are of an 

 irregular form, but where they occur in the apertures of the 

 Naticm, especially Natica glaucinoides, they present at first sight 

 somewhat the appearance of opercula*. In the other Testacea 

 in which they occur, as some species of Fusus, Pleurotoma and 

 Cancellaria, and a specimen of Nautilus imperialis, these bodies 

 are more irregularly placed, and in a few instances they were 

 found isolated in the clay surrounding the shells. 



These bodies when broken have a fibro-radiate structure of an 

 opake white, and readily yield to the knife ; they are occasionally 

 aggregated together, each one showing its centre of radiation. 

 A slight effervescence was produced upon the application of mu- 

 riatic acid, which ceased almost immediately without any appa- 

 rent diminution of the size of the specimen. 



A qualitative analysis kindly undertaken for me by Mr. James 

 Field of Loughton, Essex, yielded the following substances : — 



Silica, oxide of iron, alumina, carbonate of lime, sulphate of 

 barytes. 



Both oxide of iron and alumina were small in quantity, and 

 there was a mere trace of silica, the sulphate of barytes forming 

 by far the greater proportion of the concretions. 



The specimens were obtained from the London clay, about two 

 miles to the north of Highgate, at which place this formation 

 lies under about 10 feet of diluvium or northern drift. 



About three years since a small excavation was made through 



* The barytic substance often fills up the umbilicus of the same species. 



