the Paleozoic Bivalved Entomostraca . 421 



Dorsal margin straight or slightly depressed; dorsal angles 

 marked; extremities broadly rounded, the posterior rather less 

 obtuse than the other; ventral margin boldly convex. Valve- 

 surface compressed forwards, giving a wedge-like outline to the 

 carapace seen edgeways ; it is also turned inwards at the mar- 

 gins, thus being flattened at the margins, particularly on the 

 ventral aspect, where the angle of deflection is marked by a 

 slight ridge ; a delicate rim also accompanies the terminal and 

 ventral edges. Between the marginal angle above mentioned 

 and the border itself, there is, at the posterior extremity of each 

 valve, a sharp crest, directed upwards and inwards to meet its 

 fellow at the postero-dorsal angle, giving a notched appearance 

 to the dorsal aspect of the carapace. The surface is smooth, 

 but bears a deep umbilical pit on the middle, rather towards the 

 dorsal line. 



In the Wenlock Limestone at Crofts Quarry, near West 

 Malvern. 



Symptoms of the marginal angle of deflection, so strong in 

 P. cristata, are seen in several Primitia, and particularly in P. 

 umbilicata and P. tersa. These three forms are nearly allied ; 

 and P. umbilicata is of medium development, and may stand as 

 the type of the subgroup they represent. 



17. Primitia tersa, nov. PI. XIII. figs. 3 a-c. 



Length T iHh r , height -H^, thickness ^nr inch. 



Carapace tumid, most convex at the hinder third, rounded- 

 oblong, one-fourth longer than high ; dorsal border straight, 

 angles pointed ; ventral border very convex ; ends nearly equally 

 rounded. Surface of valves smooth, bearing a pit or umbilical 

 depression, with irregular outline, in the dorsal region, and 

 having a slight marginal ridge, which, commencing at the 

 antero-dorsal angle, runs round the anterior extremity parallel 

 to the border, and becomes lost, or very faint, at the middle of 

 the ventral border. In the Wenlock Limestone at Crofts 

 Quarry, near West Malvern. 



18. Primitia trir/unalis, nov. PL XIII. figs. 4 a, b. 



Length ^^ = ^V, height -HHhr = ts> thickness -j-^ = ^ inch 

 (about)— 11 :8:6. 



Carapace convex, compressed towards the margins, trigonal, 

 very much like the right valve of Leperditia arctica in shape, 

 having sloping dorsal angles and protruding ventral border; 

 but it has no eye-spot, or radiating muscle-spot, and the valves 

 are symmetrical and almost equal ; indeed the right valve is 

 slightly smaller than the left, though of similar outline, instead 

 of the left being much the smaller, of different shape, and 



