105 



but the specimen is so imperfect, that a definite decision of 

 this question is impossible. 



The Coel ma-bed. One or two specimens. 



Occurrence. England: Barton Beds (Wood). 



2. Mytilus af finis J. Sowerby. 

 Plate 111, figs. 2 — 3 b. 

 1829. Mytilus affinis J. Sowerby; Miner. Conch, pi. 532. fig. 1. 

 1861. — — — ; S.Wood, Eocene Bivalves. Vol.1, p. 61. 



pi. 12. 



In a rather large piece of sandstone from the Cyrena-bed, 

 which was broken up after the return of the expedition, some 

 , specimens of a little Mytilus were found. These resemble 

 closely M. affinis J. Sow., but generally seem to be somewhat 

 smaller. The form varies a little, some specimens are decidedly 

 triangular (as in Wood: Eocene Bivalves, pi. 12, fig. 1 d.), while 

 others are more rounded las in Wood, pi. 12, fig. 1 b.); like- 

 wise the thickness seems to vary somewhat. The reason of 

 these variations is perhaps due chiefly to the various specimens 

 having been subjected to pressures in different directions. — 

 The shell itself is thin, and the inner parts are probably pearly; 

 the surface is covered by finer and coarser concentric furrows 

 and ridges; no trace is found of radiating striae. 



The best preserved specimen only measures 9 mm. from 

 the umbo to the posterior margin and 5 mm. at right angles 

 to that direction ; in another larger specimen these measurements 

 are relatively 21 mm. and 12 mm. 



The smaller specimens seem to differ only by their size 

 from Wood's description of M. affinis J. Sow. For comparison I 

 have obtained a specimen from the Headon Beds of Colwell Bay. 



The Cyrena-bed: 15 specimens. 



Occurrence. England: Barton Beds of Barton. Headon 

 Beds of Colwell Bay, Headon Hill, Whitecliff Bay, Hordwell 

 (Wood). 



