247 



Orthis Nettoanaj Rathbun, sp. nov., Plate X, figs. 7, 10 and 13. 



Test very small, with the valves unequally convex. Ventral valve sub-cir- 

 cular in outline and longer than the dorsal, owing to the greater extension of 

 the beak. Dorsal valve broadly sub-elliptical in outline, slightly straightened 

 posteriorly, with the breadth greatest across the middle, and about one and 

 one-fourth the length. Breadth at hinge line apparently about two-thirds the 

 greatest width of the test ; cardinal extremities rounded ; both valves marked 

 with very tine radiating raised lines. 



Ventral valve very convex, and most prominent just behind the middle. 

 From the beak, which is elevated and acute, the surface of the valve curves 

 slightly upwards, and then slopes to the front with a regular and gentle curve. 

 Towards the sides the slope is more abrupt and slightly convex. Hinge area 

 rather high, triangular, with a large fissure. 



Dorsal valve slightly convex, broadly flattened in the middle, and often more 

 or less depressed along the median line, in a wide, undefined sinus, extending 

 two-thirds the length of the valve or more from the front, with its width iu 

 front one-third to one-half the greatest width of the valve. Beak depressed. 



The impressions of the dental lamellae iu the ventral valve diverge slightly 

 in extending forward, the distance between them being about one-fifth to one- 

 fourth the width of the valve, and their length, about one-fourth that of the 

 valve. The socket plates in the dorsal valve left similar impressions. 



The raised lines, ornamenting the valves, are exceedingly fine, rounded and 

 thread-like, closely arranged together, and seem to increase both by intercala- 

 tion and bifurcation. 



The largest ventral valve measures, length and breadth, each about 6 m. m., 

 height nearly 2 m. m. The largest dorsal valve has a breadth of 7.5 m. m., 

 a length of about 5 m. m., and a height of nearly 1 m. m. 



This is a very smull species of Ortliis, being of about the same 

 size as Orthis lepidus of the Hamilton gronp, but differing totally 

 from it in shape. It can be easily distinguished from the young of 

 Strejjtorhynchus Agassizii, with which it is associated, by the much 

 finer radiating, raised lines, and by the extension forward of the 

 dental plates in the ventral valve, and the socket plates in the dorsal 

 valve. 



From the Devonian sandstone of Erere, where it is moderately 

 abundant. (Morgan Expeditions 1870 and '71.) 



Dedicated to Dr. Ladislau Xetto, the distinguished director of the 

 Muzeu N^acional at Rio de Janeiro. 



