22 THE ENTOMOSTRACA OE 



blunt spines on the ventral moiety. Dorsal border slightly elliptical ; ventral straight, 

 strongly keeled, acuminate posteriorly. Central tubercle nearly obsolete. Hinges 

 strongly developed. 



Dorsal and ventral aspects sagittate ; anterior triangular. 



This rare species, which differs from C. cornuta, Roemer, chiefly in its want of a 

 serrated border, is figured and described by M. Bosquet as occurring at Maestricht. 

 We have met with it in the Charing Detritus, and in the Chalk of a few localities. 



Sub-genus, Bairdia, M'Coy. 



Cythere, Munster. 1830. Jahrbuch fiir Mineral. &c. 

 Cythekina, iZcBJwer. 1838. Jahrbuch fiir Mineral. &c. 



— — 1840. Verstein. Norddeutsch. Kreideb. 

 Bairdia, M'Coy. 1844. Syn. Cbaract. Carb. Limest. Foss. Ireland. 

 Cytherina, Reuss. 1845. Verstein. Bohmisch. Kreidef. 



— Bosquet. 1847. Mem. Soc. Royale Lifege. 



Sub-generic characters. — This section is very distinct from either of the foregoing, 

 both as to its form of carapace and its method of hingement. The valves externally 

 are convex and smooth, sometimes finely pitted or spined, never ribbed or granulated. 

 The hinge is simple, no bar or teeth similar to those of Cythere proper being developed. 

 Bairdia is characterised, as regards the carapace, by a somewhat similar formation of 

 valves to that which obtains generally amongst the recent Cyprides, and partially in the 

 Candonce. 



The valves frequently have more or less acute extremities, especially posteriorly ; 

 in shape, however, they vary from a triangular to an almost cylindrical form. The 

 left valve is the largest,^ and strongly overlaps the right valve on the dorsal and 

 ventral borders ; the smaller valve not fitting into a groove on the margin of the 

 larger valve, but merely lying within it. The edges of the valves are thin and 

 trenchant ; they are bevilled off on the inside of the anterior, inferior, and posterior 

 margins in the direction of the centres of the valves, and are provided with narrow, 

 lamelliform plates, casing the inside of these margins. At the anterior and posterior 

 extremities these plates frequently project so freely into the interior of the shell, that 

 considerable cavities exist between them and the inner surface of the valves. The 

 central third of the dorsal edge of the right (small) valve is straight and thin, finely 

 but irregularly serrated, and somewhat sunk in or cut out from the rest of the edge. 

 This straight portion of the dorsal margin is received in a slight depression under the 

 curled dorsal edge of the larger valve. 



1 B. Siliqua is an exception to this rule. 



