MARYLAND GEOLOGICAL SURVEY 107 



line very gently arcuate and the dorsal angles generally obtuse and 

 sometimes scarcely distinguishable. Left valve with Lhe dorsal half 

 of the outline more arcuate than the ventral, the latter sometimes 

 being even a trifle sinuate near the middle. Posterior extremity of 

 both valves generally with a rather faintly defined border. Surface 

 porcellaneous and sometimes appearing quite smooth, but when the 

 preservation is obviously good it usually exhibits numerous minute 

 punctures, generally arranged in four to six curved series, over the post- 

 central region of each valve. A small central spot, often slightly 

 depressed, is usually distinguishable by its darker color. Hinge teeth 

 rather strong in right valve, the anterior one the weaker. In the left 

 valve here is a large socket only at the posterior end of the hinge and 

 a small tooth and socket at the anterior extremity. 



The small size and shape of the valves of this very common species 

 are so distinctive when compared with associated species of the genus 

 that little trouble is likely to be experienced in their recognition. 

 Some of the species of Cylheridae, especially C. suhovata of this report, 

 might be confused with Cythere porcella, but it will require only a 

 glance at the hingemeut to see that they have no true relation to each 

 other. 



Occurrence. — Calvert Formation. Plum Point. Also at Yorktown, 

 Va., in the Chesapeake Group. 



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



Cythere nitidula n. sp. 



Plate XXXVI, Figs. 21-23. 



Description. — This neat species is not nearly so common as C. por- 

 cella, and at present the typical variety is known only from five or six 

 left valves. These, however, are very constant in their peculiarities 

 so that we cannot doubt they represent a distinct specific type. Com- 

 pared with C. porcella, which they resemble more than any other species 

 known to us, they are distinguished by the constant development of a 

 well defined flattened border margining both ends. Then the posterior 

 outline is blunter and differs particularly in the postcardinal region 

 which is more prominent. Finally, the posterior margin is not spin- 



