Jaekel on Orthoceras. — Rnedemann. 207 
one possesses four, another six, the third eight, Phragmoceras 
.and Gomphoceras however but two sinuses, which obviously 
denote separate arms. One conceives, now, the development 
of the arms usually thus ; that in nautiloids numerous arms 
were present in indefinite number, and that the latter consol- 
idated in the younger types to ten (decapods) and to eight 
(octopods). It is however, according to the ontogenetic de- 
velopment of the arms of the living dibranchiates and the mor- 
phologic arrangement of the so-called head tentacles of Nau- 
tilus, much more probable that the latter are but evaginations 
of the arms, which quite likely do not correspond to the single 
arms of other cephalopods. 
"Hexameroceras osiliense shows on either side three arms 
sinuses which gradually increase in size upward from the fun- 
nel sinus. The figure presented by their arrangement may be 
directly compared with growth-stages of living dibranchiates. 
. . . While in the mature sepia the arms surround the mouth 
radially, their inception is a paired one with a distinct sym- 
metry, so that with the appearance of the last arms the mouth 
becomes surrounded and enclosed by the circle of arms. Two 
pairs are developed first and these are soon followed by the 
third. These three pairs may also here be indicated and in the 
case cited may have flanked the mouth laterally. The encirc- 
ling of the mouth, which ontogenetically takes place at a late 
stage, had here not yet been accomplished. The circle is in- 
terrupted by a hood, partly overhanging the mouth as in Nau- 
tilus, in which however the formation of the arms has been spec- 
ialized in quite different direction from that of these Siluric 
types. In the oral view of Nautilus one sees what can 
not be learned from the current representation of the animal, 
namely that the mouth in the center is flanked on both sides 
by three leaf-like compressed arms, which are provided with 
muscular, contractile, transversely striated tentacles. 
As to their ontogenetic succession of position we know un- 
fortunately nothing and one must consider the exterior and 
'Strongest as homologous to the first arm of the dibranchiates, 
the inner ones as the second and third pairs, which are more 
distant from the funnel. . . . We can not conclude that in 
these forms the funnel was, in contrast to Nautilus, open upon 
the oral side simply from the fact that [in Hexameroceras, 
