BIVALVIA. 47 



Mactra ponderosa ? Stmpso?i. 2nd Sup., Tab. VI, fig. 2. 



Mactka pondekosa, Stimpson. Shells of New England. 



Dimensions, 2 inches by If. 



Locality. Red Crag, Waldringfield. 



A specimen of Mactra has been sent to me by Dr. Reed, with the above name and 

 locality attached by (I believe) Mr. A. Bell. It is unknown to me either as recent or 

 fossil, but it deserves a representation. Its form and appearance much resemble a large 

 specimen of M. solida, and is different from M. solidissima [21. ovalis, Gould), ' Inv. 

 Massach.,' p. 53, fig. 32, but it is not very far removed from it. 



Mactra arcuata, /. Soio. Crag Moll, vol. ii, p. 243, Tab. XXIII, fig. 5 ; 1st Sup., 



p. 155. 



I omitted to point out in my first Supplemejit that this species belongs to a section of 

 the Mactrce, which the late Dr. J. E. Gray proposed to distinguish as a separate genus 

 under the name of Sj)isula, this section being distinguished by the possession of the 

 fimbriated mark or perpendicular striation on the lateral teeth, which forms part of the 

 diagnosis of this species given at p. 243 of the 'Crag Moll."; and that Mactra glauca, of 

 which arcuata is called a variety in the list Avhich accompanies Mr. Prestwich's paper 

 on the Crag, belongs to the other section, viz. that which is destitute of this impression. 



A fragment of a full-grown shell, showing the hinge with this fimbriated mark, and 

 which therefore seems to be one of M. arcuata, was obtained by my son from a band of 

 shell fragments at the top of the jNIiddle Glacial sand, three or four feet below the over- 

 lying chalky clay, in a well at Bealings, near Woodbridge, this seam exactly corresponding 

 in position to that at Billockby and Hopton, from which the species given in my first 

 Supplement were obtained. 



Thracia papyracea, Foil. 2ud Sup., Tab. VI, fig. 6 «, h, 



Thracia phaseolina. Crag Moll., vol. ii, p. 259, tab. xxvi, fig. 2. 

 — papykacea. 1st Sup. to do., p. 15G. 



Additional localities. Chillesford Bed, Sudbourn Church Walks ; Lower Glacial, 

 Belaugh. 



Dr. Reed having sent to me a specimen with the name Thracia villosiuscula attached, 



