OSTEACODA. ^^^ 



Jonesella.) 



Genus JONESELLA Ulrich. 



Jonesella Ulrich, 1890. Jour. Cin. Soc. Nat. Hist., vol. xiii, p. 121. 



Carapace small, equivalved, moderately convex, oblong-subovate; hinge straight. 

 Valves with a curved ridge on the posterior half or two-thirds. This ridge may be 

 variously modified, but in the typical species it is thin and bent like a horseshoe, in 

 another the anterior arm is horizontal instead of vertical, while in a third the two 

 arms are divided. Edges simple or faintly bordered. 



Type: J. crepidiformis Ulrich. 



Fig. 47.— a, b, c, lateral, posterior, and ventral views of an entire carapace of Jonesella crepidiformis 

 Ulrich; d, left valve of J. pedigera Ulrich; all about x 20. Both species are from the lower beds of the 

 Cincinnati group at Covington, Kentucky. 



The aflSnities of this genus, which includes so far as known only Lower Silurian 

 species, appear to be with Bollia, Jones and Holl. But the horseshoe ridge in all 

 true species of that genus is subcentral, while the edges of the valves are thickened 

 into a more or less well-developed marginal ridge, no trace of which is apparent in 

 Jonesella. The new species about to be described is peculiar in the faint develop- 

 ment of the loop, and in the shortness of the horseshoe. In J. crepidiformis, as may 

 be seen in the above cut, the ridge takes up the greater part of the posterior half. 

 Still, a general agreement of parts between the two species is obvious, so that J. 

 obseura may well be accepted as an incipient Jonesella. On the other hand, the 

 prominent upper extremities of the bent ridge, remind considerably of certain species 

 of Ulrichia, but as the whole carapace recalls Bollia quite as much, if not more, it is 

 to be assumed that these resemblances indicate family relationship rather than 

 generic. As usual with early types of life, the Lower Silurian Ostracoda are apt to 

 be of a composite nature, and the determination of the really significant features of 

 such forms, so far as generic and specific alliances are concerned, is never certain 

 except through minute genealogical investigations. But this touches upon too large 

 a subject for the present work. 



In the original work on the genus I included a Minnesota species, under the name 

 of J. crassa, that I shall now place elsewhere, because it seems to belong to another 

 line of development, namely, it is closely related to Ctenoholhina fulcrata. For 

 further remarks on this and related species see under Ctenoholhina. 



