924 Report or toe Stare Groroaist. 
of solidification of the brachia in this group is to be regarded as 
an index of differentiation. To illustrate: There is no evidence 
for assuming that the single revolution made by the spiral in 
Prorozyea and Haturna represents an incomplete spiculation of 
the brachia, or that the spiniform and discrete jugal processes in 
SpirireR, persisting throughout the genus, do not fully exemplify 
the adult condition of the jugum (-loop) in these shells. The 
mode of spiculation of the brachia in such of the living terebratu- 
loids, in which the solidification is direct or without complicated 
metamorphoses, is on the whole confirmatory of this inference ; 
but as there is no living representative of the spire-bearing forms 
evidence in regard to the mode and degree of spiculation in this 
group to be derived from the existing loop-bearing shells, in 
which the brachial supports pass through highly complicated 
metamorphoses, is not altogether germane In such intricate 
structures as the brachidia of Atuyris, Kaysprra, Koninckina, 
etc., there can be little doubt that the calcified apparatus repre- 
sents the full extent of the fleshy brachia simply because, if for 
no other reason, the further expansion of the brachial lamelle 
would not be possible for want of space. Moreover, in the spicu- 
lation of the spirals in all these old shells there have been no 
changes of form in later growth except those proceeding from 
the normal process of resorption and deposition necessary for 
Increase in size and length. The reason why the spiculation 
should be complete in the spire-bearing forms, while in the Anoy- 
LospRacHia or the terebratuloids it does not extend beyond the 
loop and the lateral extensions of the brachia, but in the Rarn- 
CHONELLID# affects only the crura, and in the SrropHOMENIDa 
does not occur, even in the most elementary condition, is for 
future investigations to ascertain. 
The form of the paired spirals varies but little except under 
the necessity of conforming to the intcrior cavity of the valves. 
Their inclination and direction is a feature of much significance 
when considered with reference to the development of the entire 
shell. It is, however, the loop, or to employ a term more appro- 
priate in view of the homologies of the spire-bearing and loop- 
bearing shells, the yugwm, which is subject to the most frequent 
variations in form, and which serves as the generic index. When 
the spirals are directed outward toward the lateral margins of the 
176 
