OSTEACODA. 685 



Oytherella? subrotunda.] 



these appear to be within the limits of the genus. Prof. Jones has also referred 

 several Lower and Upper Silurian species to the genus, but here, it seems to me, the 

 generic relations are in every case at least doubtful. The following two species at 

 any rate, are almost certainly not Oytherella, yet' they must be placed here because 

 their known characters are more in accordance with this genus than with any of 

 the others that have been established. 



At present the principal diagnostic feature of Cytherella, that is, with the 

 paleontologist, is the rabbeted edge of the right valve. This peculiarity, if my 

 memory is not at fault, has not yet been shown to exist in any of the Silurian 

 species hitherto referred to the genus. It does however exist, and very strongly 

 developed too, in an undescribed species from the lower beds of the Cincinnati group. 

 In this spedies, however, unless all the specimens seen (about twenty) are of one 

 valve only, the edges of both valves are about equally grooved. 



For remarks on Cytherellina, Jones and Holl, see under Bythocypris and B. 

 cylindrica. ' ■ 



Oytherella. ? subeotunda, n. sp. 



PLATE XLIV, FIG. 43. 



Size.— Length 0.5 mm.; hight 0.45 mm. 



This species is founded upon a single carapace attached to the surface of a 

 fragment of the zoarium of Pachydidya foliata. The smaller valve is exposed to 

 view, and around it the overlapping edge of the larger, presumably the right valve, 

 is distinctly defined. The outline is broad-oval, almost circular, and as near as can 

 be determined, the surface of the smaller valve is moderately and quite uniformly 

 convex, and exhibits neither a central .depression, a tubercle, nor markings of any 

 kind. The specimen was found in association with valves described on a preceding 

 page as Schmidtella? suhrotunda. They are distinguished by a small, subcentral 

 tubercle, but as they have the same rounded outline, it is possible that a better 

 preserved series of specimens may show them all to belong to one species. That the 

 synonomy may, in case the possibility is converted into a fact, be simplified, I -have 

 used the same specific name for both. 



Formation and locality. — Lower third of the Trenton shales, Minneapolis, Minnesota. 



