14 THE ENTOMOSTRACA OF 



Subgenus Cythereis, nobis. 



Synonyms. Cythere, Miinster. 1830. Jahrbuch flir Mineralogie, &c. 

 Cytheeina, fiamer. 1838. Jahrbuch fiir Mineralogie, &;c. 



— _ 1840. Versteiu. Kreidberg. 



— Reuss. 1845. Verstein. Bohm. Kreideform. 

 Cythere, Cornuel. 1846. Mem. Geol. Soc. France. 

 Cytherina, Williamson. 1847. Transact. Manchester. 

 Cypridina, Bosquet. 1847. M6m. Soc. Royale Lifege. 



The carapace-valves of Cythereis have an almost regularly oblong shape. The 

 superior (dorsal) and inferior (ventral) borders of the valves lie nearly parallel to each 

 other ; the superior border, however, especially in the left (larger) valve, trends up- 

 wards as it approaches its anterior extremity, making at its junction with the anterior 

 border a more acute angle than that formed by the junction of the anterior and 

 ventral borders, and thereby leaving a greater space between the anterior hinge 

 and the ventral margin, than between the same margin and the posterior hinge. The 

 middle of the inferior border is slightly incurved. The anterior border is nearly 

 semicircular, it is trenchant and bevilled off inwardly ; its superior moiety is more or 

 less compressed, and, as it were, drawn back to meet the dorsal border. The anterior 

 third of the carapace-valves is always depressed. The posterior border is shorter 

 than the anterior ; it is more or less acute, its ventral moiety forming the segment of 

 a circle, and the edge of its superior moiety suddenly returning in a straight or sinuous 

 line to meet the dorsal border. This somewhat triangular extremity, occupying some- 

 times a fourth of the valve, is much below the level of the rest of the surface, being 

 suddenly and strongly depressed. The inferior or ventral moieties of the anterior 

 and posterior borders are always furnished with spines, and sometimes nearly the 

 whole of the anterior border. 



The dorsal half of the valve, or that superior to the median line, has always less 

 convexity than the ventral or inferior half, which latter is frequently very strongly 

 raised, taking an angular form, with the ventral surface of the valve at a right angle 

 to its lateral surface ; and as the ventral half of each valve gradually increases in 

 thickness, as it recedes from the anterior extremity until it is abruptly terminated 

 nearly opposite to the posterior hinge, the ventral aspect of the carapace is flat, and 

 more or less of a triangular or sagittiform shape, varying, indeed, from an irregular 

 oblong more or less elongate to a triangle, the posterior angles of which are sometimes 

 far apart, and giving origin to Dr. Reuss's group of " Cornutce." The dorsal aspect, 

 on the contraiy, is not fiat, but angular or culminate, narrow in front, and increasing 

 in width backwards towards the ventral keels. For the same reasons the anterior 

 aspect of the carapace is generally triangular, acute superiorly, and inferiorly more or 

 less extended. 



