18 NEW UNIONID.E OF THE 



The form of the branchial ovisack is club-sliape, tapering toward the superior end and 

 blunt at the lower one, which is terminated with a knob, and covered with a translucent 

 membrane. These ovisacks are about three-tenths of an inch long and one-thirtieth 

 wide,* attached by the point to the middle of the outer branchial leaf, the line of attach- 

 ment running nearly the whole length in a zig-zag manner, which gives a folded form 

 to the whole branchial uterus, the ovisack extending below the lower edge of the 

 branchia. These ovisacks are blackish from the middle down to the end, where the 

 knob-like termination is perfectly white, so that on looking at the ends of the folds, 

 the whole edge has the appearance of a string of pearls. Under the microscope may 

 be seen at the end of each sack a white bell-shaped substance in the interior. These 

 folds cause the length of the lower edge of the outer branchice to be quite three times 

 the length of the edge of the inner branchiae. In my largest specimens the number of 

 folds is sixteen on each of the outer leaves of the branchise. See pi. 29, fig. 103. 

 Branckice free quite half the length of the^abdominal sack.-j- The inner branchiae much 

 larger than the outer ones. They are all large and semicircular below. Paljn small 

 and suboval. Mantle thin, with a delicate border all round, slightly colored. Branchial 

 opening rather large with numerous, thickly set, small brownish papillae. Anal 

 opening large with numerous very small brown papilla on the inner edges. Super-anal 

 opening rather long, colored on the inside, and very slightly attached below. 



Embryonic Shell is elongate pouch-shape. J 



Remarks. — This is a remarkably interesting sjiecies owing to its rare form oihranchial 

 uterus, which is constructed on the same plan with U. phaseolus, as mentioned above; 

 but being a much smaller species the number of folds of the uterus is by no means so 

 great. In outline this species is nearly allied to Formanianus (nobis), but it is more 

 triangular. It differs also in the rays which are maculate and not capillar3\ It is 

 also nearly allied to phaseolus in its general outward phase, being yellowish and 

 maculate, but it is by no means transverse nor so flat-sided as that shell. It is to be 

 regretted that the soft parts of the Formanianus and several allied species,. viz.: Greenii 

 Con., &c., have not been observed, as they are likely to possess the same peculiar 

 branchial uterus. The teeth of Woodwardianus are remarkable for the high and 

 subangular arch formed between them. This character is very striking. It is with 

 pleasure I dedicate this very interesting mollusc to S. P. Woodward of the British 

 Museum, author of the excellent " Treatise on Recent and Fossil Shells." 



Unio denigratus. pi. 23, fig. 83. 



Testa Isevi, oblongti, subinflata, ad latere planulata, insaquilaterali ; valvulis subteuuibus, antice crassioribus ; 

 natibus prominulis, ad apices undulatis ; epidermlde nigrieante, obsolete radiata et micanto ; dentibus 



*See ph 5, fig. ICa. 



t One specimen out of \i was free only a short distance. 

 L {See pi. 5, fig. 16. 



