MOLLUSCA. 527 



margin rather short, obliquely subtruncate, straight or slightly 

 convex ; posterior cardinal slope more concave than the anterior. 

 Valves ventricose in the umbonal region, the surface curving 

 regularly to the margin all around, being more abrupt to the 

 cardinal margin, and often somewhat compressed towards the 

 postero-cardinal extremity; slightly gaping behind. Surface 

 marked by 25 to 30, and in very large individuals as many as 

 35, angular, radiating costae with concave interspaces, a small 

 area at the posterior extremity being nearly or wholly destitute 

 of ribs. External impressions of the shell show these ribs to be 

 crossed by fine concentric lines of growth, and to be surmounted 

 along the summit by a row of small tubercles appearing almost 

 like spine bases, whose distance apart is less than the distance 

 between adjacent costae; the radiating rows of tubercles also 

 continue across the posterior noncostate portion of the shell. 



Remarks. This is the commonest member of the 'genus in 

 New Jersey and occurs, so far as known, only in the form of 

 internal casts with their external impressions. The delicate sur- 

 face markings of the shell can only be seen in impressions of the 

 external surface, and only occasionally in these because of the 

 imperfection of their preservation. Whitfield's Z,. innata is cer- 

 tainly a synonym of this species; it is said to be shorter, with 

 more inflated beaks, and less sharply defined costae, but these 

 differences are nothing more than individual variations. 



Formation and locality. Wenonah sand, near Marlboro 

 (130); Navesink marl, Atlantic Highlands (108), near Craw- 

 fords Corner (i26 7 ), near Walnford (i4& 2 , 149), Crosswicks 

 Creek (195), Mullica Hill (169) ; Red Bank Sand, near Middle- 

 town (112), Red Bank (116), Shrewsbury River (119) ; Tinton 

 beds, Beers Hills cut, south of Keyport (i29 5 ). 



Geographic distribution. New Jersey, Alabama, Mississippi, 

 Arkansas, Texas. 



Liopistha alternata n. sp. 

 Plate LVIIL, Figs. 7-9. 



Description. The dimensions of an average left valve are: 

 length, 22 mm.; height, 15.5 mm.; convexity, 7 mm. Shell, 



