20 THE ENTOMOSTRACA OF 



shells the punctations are encroached upon by the intervening raised parts, which 

 latter become more and more augmented, until the surface is roughly reticulated, and 

 covered with granulations and spines, which are more or less obliquely arranged, 

 especially on the posterior half. Central tubercle strongly developed ; in old valves it 

 is four-cleft. From the central tubercle of young shells an interrupted ridge runs 

 backwards, which becomes gradually lost in old shells amidst the granulations of the 

 surface. Anterior extremity obliquely rounded, shelving ; posterior suddenly depressed 

 into a short, flat, bearded lip, acute in young valves, and becoming more obtuse with 

 age. Dorsal and ventral borders nearly straight. Hinges strongly developed. 



Dorsal and ventral aspects vary according to age ; in the young shell, owing to the 

 comparatively greater prominence of the tubercle, and the less development of the 

 ventral ridge, the profile is somewhat fiddle-shaped, but it gradually approaches with 

 age to a sagittate form. Ventral aspect of each valve in young shells presents 3 — 4 

 longitudinal rows of punctations, which in the old shell are replaced by three rows 

 of granulations. Anterior aspect triangular ; at first lobed and contracted, ultimately 

 forming an equilateral triangle. 



This species is present in most of the members of the Chalk Formation, but is most 

 abundant in the Gault. The Detritus affords the largest individuals. Dr. Reuss 

 describes the immature form as C. ciliata, from the Bohemian Chalk-marl, and Mr. 

 Williamson has noticed the adult form as occurring in the Detritus of Charing. 



No. 7. Cythereis Lonsdaleiana, nobis. Tab. V, fig. 12 a—c. 



INCH. 



Length, -^^ Chalk, Norwich. 



Height, ^V 



Thickness, -^-^ Upper Oolite, Dorset. 



Carapace-valves oblong, rather depressed, especially on the dorsal half, smooth, 

 rounded anteriorly. Posterior extremity rather contracted, obtuse, bearded. Dorsal 

 margin nearly straight ; ventral incurved at its anterior third. A thin, smooth semi- 

 circular ridge lies near the anterior margin, and a similar slightly curved ridge runs 

 along near the ventral border. On the dorsal border are situated 2—4 short, curved, 

 crenulated ridges, lying obliquely, anteriorly pointing towards the median line of the 

 valve, and posteriorly terminating on the dorsal edge. Central tubercle prominent. 

 Hinge-teeth strongly developed. 



This species is rare. It occurs in the Soft Chalk at Thorpe, near Norwich, and is 

 present also in the White Limestone with flints of the Upper Oolite. 



We have dedicated this interesting species to Mr. Lonsdale, who first pointed out 

 the existence of Cythere in the Chalk of England. 



