245 



his geuerons aid towards fitting out three different expeditions to 

 Brazil, and for. his constant, warm sympath3Mvith my scientific pur- 

 suits.— C. F. H.] 



Retzia Wardiaiia, Hartt, sp. uov., Plate X, figs. 2-5, 8, 1), 11, 12, 14 and 16. 



Test small, double convex, more or less ventricose, the ventral valve being' 

 usually the more convex ; longitudinally suboval, slightly angular behind ; 

 usually a little longer than wide, though the width sometimes equals the 

 length, greatest width near the middle ; Avhole surface finely plicated. 



Ventral valve most convex near the center, curving regularly and moder- 

 ately, sometimes quite strongly, from beak to front, and very strongly from 

 side to side. Beak more or less pointed and slightly incurved. Posterior lat- 

 eral margins of valve straight, slightly convex or concave, including an 

 angle equal to, or a little more than, 90 degrees. The front and lateral mar- 

 gins together form a little more than a semicircle. The bases of the dental 

 plates are parallel and extend very slightly forward, the distance between 

 them being nearly one-fourth the width of the valve. 



Dorsal valve nearly circular in outline, sometimes a little elongate, generally 

 slightly and regularly convex, the beak being much depressed. 



Each valve is ornamented by from 14-20 (in one case 22,) simple, narrow, 

 rounded, or subangular, very prominent, radiating plications which extend 

 over the whole surface, being traceable from the beak, where they are very 

 narrow^, to the front, towards which they gradually increase in width and prom- 

 inence. These plications do not differ much in size, being only a little smaller 

 towards the lateral margins than in the middle. The depressions separating 

 the plications are narrower than they, and rounded or angular. 



A medium sized ventral valve measures, length 10 m. m., breadth 8 m. m., 

 thickness about 3 m. m. 



This species occurs quite abundantly in the Devonian sandstones 

 of Erere, associated with R. Jamesiana, which last is, however, much 

 more common. The two species resemble one another closely in 

 size and general form, but they differ totally in ornamentation, 

 the difference being so marked that the species are readily distin- 

 guished, even by fragments of valves ; moreover, no intermediate 

 forms have been observed. R. Jamesiana always has low, indistinct 

 plications not extending to the beak, while those of the species just 

 described are always very prominent, extending from i\\Q beak to the 

 front. The enlarged median plication and depression are also a 

 constant character of R. Jamesiana. (Morgan Expeditions 1870 

 and '71.) 



