exceedingly tumid and well-nigh of globular form. The greatest 

 width lies rather behind the middle and is considerably greater 

 than the height, or about equalling 4 /o of the length. The an- 

 terior extremity is obtusely pointed, whereas the posterior is evenly 

 rounded, its contours forming with the lateral ones quite an 

 uninterrupted curve. The dorsal part of the shell is in the 

 posterior half distinctly depressed along the median line, forming 

 here a rather deep groove. The surface of the shell exhibits a 

 dull granular aspect, on account of numerous small depressed 

 pits, and is rather densely beset with hair, more especially at 

 both extremities. 



The valves are subequal and rather thin, except tne margi- 

 nal part of the anterior extremity, which is highly chitinised 

 and provided with very conspicuous distant transverse grooves 

 see PI. VII, fig. 1). This marginal part is surmounted by a 

 very thin and pellucid border, and a similar, though less devel- 

 oped border also occurs at the posterior extremity. The edges 

 of the right valve are quite smooth, whereas those of the left 

 exhibit posteriorly a series of very small, tuberculiform denticles 

 (see fig. i). 



The colour of the shell is light yellowish green, with 3 

 rather conspicuous, irregularly flexuous transverse bands of a 

 very dark greenish hue. Of these bands the posterior is rather 

 narrow and well defined from the other two, which on the 

 other hand, coalesce above the impressions of the adductor 

 muscle to a larger, irregular patch. The foremost of the two 

 gives anteriorly origine to a short lateral branch, which how- 

 ever does not reach to the anterior edge. The marginal part 

 of the anterior extremity is very dark-coloured, almost black, 

 but is surmounted by a narrow, light-coloured border. The shell 

 °n the whole is not very pellucid, and the only inner organ, 

 w &ich is distinctly traced through the same, is the eye. 



The latter organ (see PI. VII, figs 1 and 2) is located 

 rather close to the dorsal margin and, when viewed from above 

 (see also PI. II, fig. iq) appears very broad, the ocular pigment 

 having the form of a short cylinder placed transversally to the 



