OSTRACODA, 667 
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, 6, ¢, lateral, posterior, and ventral views of an entire carapace of Jonesella crepidiformis 
Ulrich; d, left valve of J. pedigera Ulrich; all about = 20. Both speciesare from the lower beds of the 
Cincinnati group at Covington, Kentucky. 
The affinities of this genus, which includes so far as known only Lower Silurian 
species, appear to be with Bolla, 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 includeda 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 Ctenobolbina fulcrata. For 
further remarks on this and related species see under Ctenobolbina. 
