244 



simple, radiating plications, separated by slightly narrower, rounded depres- 

 sions. The plications, which are very distinct at the front, extend backwards 

 from one to two-thirds the length of the valve, when they disappear, leaving 

 the entire unbonal region smooth. The depression, occupying the median 

 line, is about twice as wide as the others, but only slightly deeper, and extends 

 nearer to the beak. It is flattened along the bottom, and very rarely includes 

 a slight fold in the middle. The plications on the sides of the valve do not 

 vary much in size, those bordering the central depression being only slightly 

 larger than the others. Thin dental plates, starting on each side of the beak, 

 and diverging but slightly, extend forward along the valve for about one-fifth 

 its length. 



Dorsal valve sub-circular in outline, sometimes a little angular behind, 

 more or less depressed-convex, sometimes very much depressed, with the great- 

 est elevation behind the middle. Beak sharp, its margins forming nearly a 

 right angle, dej^ressed, and appearing not to project beyond the hinge line. 

 The plications of this valve correspond in number and character to those of 

 the opposite valve, being distinct on the front, while the posterior part of the 

 valve is smooth. The plication occupying the median line is slightly enlarged, 

 corresponding to the median depression in the ventral valve. It is flattened 

 on the top, scarcely more prominent than other plications, but extending 

 nearer to the beak. 



The largest specimen obtained, a ventral valve, measures, length 11m. m., 

 breadth 10 m. m., height 3 m. m. 



This species occurs very abundantly in the Devonian sandstone 

 at Erere, associated with Streptorliynclius Agassizii, etc. The area, 

 deltidium and loop, are not preserved, but from external features it 

 appears to approacli more nearly to Retzia {Rhyncliospira) lejiida, 

 Hall, Hamilton group, than any other species; but I have no speci- 

 mens for comparison. In ornamentation it resembles Retzia radia- 

 Us, Phil., Carb., Eng. The plications in R. Jamesiana, however, 

 are smaller, but the median plication on the dorsal valve, and the 

 corresponding sinus on the ventral valve, are enlarged as in R. 

 radicdis. 



The extension of the dental plates to the bottom of the ventral 

 valve in the new species, is very distinctly shown in some of the 

 interior moulds, a character which, hitherto, has seldom been found 

 in any species of Retzia ; but it is not safe to decide on its A'alue 

 until the interior of Retzia is better known. (Morgan Exj)editious 

 1870 and '71.) 



[In dedicating this species to my old friend, Maj. 0. C. James, of 

 Rio do Janeiro, I desire to express my deep feeling of gratitude, for 



