154 



Seldom do we find any aggregations of individuals, and never the swarms 

 on a single layer of rock that may be found in occurrences of the later 

 Ostracods ; hence they appear not to have possessed in any marked degree 

 the gregarious habit of these later genera. 



Another peculiarity of the Etcheminian and Protolenian forms, as 

 distinguished from the prevalent Ostracoda of Ordovician and Silurian 

 Time, is the unusual convexity of the front moiety of the valve as com- 

 pared with the other. This for some time led the author to be uncertain 

 as to which was the anterior end of the valve in the genus Beyrichona. 

 He, however, now has no longer any doubt, as the related genus Bradoria, 

 with its prominent ocular tubercle sets this matter at rest, and shows 

 that the thick end of the valves is the anterior one. 



Another common feature is the prevalence of species which are as wide, 

 or wider than long. This peculiarity is connected with a long hinge 

 line and with more or less abrupt cardinal curves of the margin, 

 before these merge into the true anterior and posterior margins of the 

 valves (see Plate I. figs. 1 to 6, a & 6). When the angle at the lower end 

 of these cardinal curves is acute, a tubercle is sometimes developed, in 

 addition to that which marks the anterior and often the posterior end of 

 the hinge line (PI. I. fig. 13, a & c). 



These laterally expanded valves are in a number of species somewhat 

 pointed at the lower margin, and in most the ventral margin is more or less 

 angulated (PL XII. figs. 2, 3 and 6). It thus admits of division into two 

 portions which may be designated the anterior and posterior curves (e & d). 

 according to whether the part of the margin indicated is in front or 

 behind the ventral angulation. Sometimes the anterior curve of the mar- 

 gin will be stronger as in Beyrichona (Fig. 3) (and Escasona ? Fig. 6), 

 sometimes the posterior curve, as in Indiana (Fig. 1) and Bradorona (Fig. 

 2), is the stronger. In Hipponicharion (Fig. 4) the two are about equal. 



Terms used in The cardinal curves (PI. XII. figs. 2 etc., a and 6), extend from the 



theseOstra- hi n & e li ne along the margin until it becomes at right angles to the hinge, 



coda. and they also vary greatly in direction and extent. Thus in Indiana 



(Fig. 1) the posterior one is long, the anterior shorter; in Bradorona 



(Fig. 2) they are approximatively of equal length ; in Beyrichona (Fig. 3), 



sp. palpilio, the posterior one is almost obsolete, but in other species 



(tinea, planata, etc.,) of this genus, it is well shown, and with these the 



species of Escasona (Fig. 6) agree. In Bradoria (Fig. 5) both cardinal 



curves, and especially the posterior, are well shown. In Hipponicharion 



on the contrary these curves are almost obsolete. 



