1 84 



by equally wide interspaces. Along the anterior border the rib is about r 

 mm. wide. In the sinus there are two or three narrow ribs in alternation 

 with interspaces nearly twice as wide as the ribs themselves, which mea- 

 sure about O.S-O.S mm. in breadth at the anterior extremities. When there 

 are three of them in a sinus the one at the middle is the widest. The two 

 ribs forming the boundar\' between the sinus and the lateral parts look extra- 

 ordinarily high because of the depth of the sinus in the front region. The 

 ribs are simple and seem never to increase in number anteriorl)^ On the 

 median fold of the dorsal vah e. The ribs are usuall}- wide and the inters- 

 paces narrow. The growth lines are not very distinct but the)- really ex- 

 ist and form fluctuating lamellae, which are convex in the intercostal spaces 

 and concave on the ribs. Shell is ver\- thin. 

 Dimensions : — 



Length. W'idtli. Thickness. Hinge-line. 



13 mm. 13 mm. 9 mm. ' 5 mm. 



14 15 >. 10 ,, 7 



15 .■ 15 n II „ 7 » 



15 „ 15 „. 12 „ ' 5 



Remarks : — Within this species the range of variation is somewhat re- 

 markable with regard to morphological features. The anterior prolongation 

 of the sinus is in some examples very long, while in others it is not so conspi- 

 cuous. Then, too, the number of the radial ribs is b)' no means constant. 

 In some examples S of them may be counted on one side of the shell, 

 although, on the other hand, some have only 6. 



In the younger examples the sinus and fold are very indistinct, but in all 

 other points the specific identit}- is not a matter of any great difficulty. In 

 such small specimens the ribs are very distinctly developed, so that they 

 might be called plications rather than ribs. 



P^rom the preceding forms this is descriminated first of all because of the 

 small size of the shells. There is .not one among quite a large number of 

 specimens which equals the preceding forms in point of size. The smallest 

 examples of the other forms are separable from the present b)^ their 

 peculiar shape and the characteristic features on the surface of the shells. 



