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thin, straight, and projecting, and the mouth widely open. 

 In H. ve?itricosa the lip formed by the upper valve is very 

 much curved, so as to come quite over and hide the narrow 

 mouth, and in H. digitata the lips are thickened and everted. 



2. Hyalea ventricosa n. sp. (PI. vii., f. 15). 



Almost globular, rounded and much inflated in front ; 

 lower valve swollen ; upper valve spoon-shaped, inflated, 

 adorned by three keels or ribs radiating from the umbo to 

 the extremely reflected and recurved lip ; the central keel 

 more pronounced and distinct; the lateral ones broader and 

 more rounded; lateral mucrones simply angular, not pointed, 

 terminal mucro short and pointed ; mouth narrow, hidden 

 by the protusion of the sharp thin lip. Lenth 4^ mill., 

 breadth 4 mill. 



Bears some resemblance to H. globulosa Rang and to H. 

 quadridentata I^es. in general character. In our shell the 

 lateral mucrones are single, and there is a decided terminal 

 mucro ; the central rib is narrower and more defined than 

 the broader rounded one on each side of it. 



3. Hyalea digitata n. sp. (PI. vii., f. 16). 



Oblong, rather compressed ; lower valve inflated ; upper 

 valve slightly convex, adorned with three nearly equal 

 rounded radiating ridges, constricted in front, and bordered 

 by a thickened lip ; mouth narrow, lips thickened and 

 everted ; terminal mucro short, sharp ; lateral mucrones 

 scarcely produced. Length 5 mill., breadth 3 mill. 



In some respects resembles H. tridentata Bosc, and H. 

 uncinata Rang, but is not so inflated, and has shorter and 

 less curved lateral mucrones. 



