Report of the State Geologist. 



519 



their greatest development, is always devoid of this support, and 

 equally noteworthy that this genus, which seems poorly adjusted 

 against such strains when compared with other spire-bearing 



Fig. 152.— Jugum of Meristella arcuatn. 



FlQ. 153.— Jugum of Xueleos-jiira ventricoaa . 



genera, should have maintained so long and so prolific a develop- 

 ment without variation in this respect. 



The jugum does not end at the point of union of the two 

 lateral jugal processes, but is more or less continued, in its 



Fig. 154.— Jugum of Aihyris vittata. 



simplest form, into a short, straight stem, inclined backward and 

 terminating inanacute angle (Retzia, Rhynchospira, Whitfield- 



Brachial apparatus of Bifida lepida. 

 Fig. 155.— Longitudinal section. Fig. 156.— Transverse section. 



ella) ; in Bifida this stem is long and extends to the inner 

 surface of the pedicle-valve where its extremity is inserted into 

 a deep groove in the shell. In Eumetria and Mkristina the 



71 



