116 SYSTEMATIC PALEONTOLOGY 



left valve as well as the right and in being at least one-fifth larger. Com- 

 pared further with average specimens of C. martini, the posterior ex- 

 tremity is much more produced and compressed, while the marginal 

 spines are coarser. The surface markings are also coarser, but the ante- 

 rior swelling is relatively smaller. The specimens exhibit some variety 

 in the relative height of the posterior end, but as a rule it is nearly or 

 quite as great here as across the middle of the valve. 



Bosquet figures several related European Tertiary species but none is 

 deemed close enough to require more than ordinary care in discriminat- 

 ing our species. It appears to be one of the most elongate of its section 

 of the genus. In this respect as well as in its outline it is almost matched 

 by C. venustula described by Jones and Sherborn from the Eocene of 

 England, but in all other external features the two forms are very differ- 

 ent, so that it is doubtful even that they have any close genetic relations. 



Length of left valve 1.05 mm., greatest height of same 0.47 mm., 

 length of right valve 0.90 mm., greatest height of same 0.40 mm. 



Occurrence. — Calvert Formation. Plum Point. 



Collections. — U. S. National Museum, Maryland Geological Survey. 



Cythere micula n. sp. 

 Plate XXXVI, Figs. 18-30. 



Description. — This species also may be said to be rather closely related 

 to C. martini, bu.t it is very constant in its peculiarities and clearly de- 

 serves specific recognition. Closely compared with that species it is found 

 to be constantly of much smaller size, with the posterior end relatively 

 narrower, the marginal rim much thinner, causing the surface of the 

 valves to appear more uniformly convex, and the pitting of the surface 

 much finer. Furthermore, the surface swellings are relatively broader 

 and so placed that a slight central depression is left that has no parallel 

 in C. martini. Both ends of the left valve bear an extremely delicate 

 fringe of spines. Such spines, however, have not been observed on the 

 right valve. 



This is the smallest species of the genus known to us, and on this 

 account, though not uncommon, easily overlooked. 



Length 0.5 mm. to 0.58 mm., height 0.2T mm. to 0.3 mm. 



