INVERTEBRATE PALEONTOLOGY. 211 



Mactra (Cymbophora!) uitidula, M. & H. 



Plate 30, figs. 6, a, b, c. 

 Tellina nitidula, Meek and Haydeu (1861), Proceed. Acad. Nat. Sci. Philad., XIII, 443. 



Shell transversely subovate, compressed, very thin ; anterior side regu- 

 larly rounded ; base serai-ovate, or somewhat straightened along the middle; 

 posterior side narrower than the anterior, and narrowly rounded or suban- 

 gular at the extremity below the middle ; dorsal outline sloping from the 

 beaks, the posterior slope being longer and straighter than the anterior; 

 beaks moderately prominent, rather compressed, and placed a little in advance 

 of the middle ; posterior umbonal slopes showing a slight disposition to form 

 an obscure ridge from the beaks toward the posterior basal extremity ; sur- 

 face with fine concentric striae, and a few, irregular, stronger furrows ot 

 growth; muscular impressions very shallow ; pallial line apparently of mod- 

 erate depth, and rounded. 



Length of a mature specimen, 1.11 inches; height, 0.87 inch ; convex- 

 ity, 0.42 inch. 



In first describing this shell, it was stated that " we cannot determine 

 with positive certainty its generic characters; " nothing being then known in 

 regard to its hinge. From its extreme thinness, apparently slightly flexuous 

 posterior, and general appearance, however, it was thought probably to belong 

 to some of the less elongated sections of the genus Tellina, and referred pro- 

 visionally to the same, with the remark " that the sinus of its pallial line 

 seems to be rather short for a species of that genus." 



On a more critical examination of the specimens, I became satisfied 

 that the apparent slight lateral flexure of the posterior extremity of some of 

 them is merely due to accidental distortion; and on clearing away some 

 adhering portions of rock between the beaks, I soon found that it had no 

 external ligament like that of Tellina. Consequently, it became evident 

 that it must find a place in some other group. The difficulty, however, of 

 determining the nature of its hinge still remained; none of the specimens 

 showing the hinge-margin, while the hard rock filled the whole interior of 

 them all. With the hope of developing, at least, some clew to the nature of 

 its hinge, I ground away the rock, with much difficulty, from the inner side. 



