7110 ALLIS. [Vou. XII. 
muscular septum where that septum is crossed by the transverse 
intermuscular septa (Rabl, Goppert). In Lepidosteus, on the 
contrary, they still lie in the horizontal septum. A condition 
intermediate between these two is indicated in the anterior ribs 
of larvae of Amia, which lie buried, at their proximal ends, in 
the trunk muscles, approximately at the level of the horizontal 
septum. AQ still less advanced condition is shown in the rib- 
like ligaments which lie in front of the anterior ribs in Amia, 
for although lying below the horizontal septum they lie, even in 
the adult, entirely in the trunk muscles. 
The horizontal muscle septum is said to be a relatively late 
acquisition (Rabl, quoted No. 48, p. 166). Being such, one 
would naturally suppose that the “ Anlagen”’ of the upper ribs 
would lie in the transverse septa, immediately ventral to the 
horizontal septum, rather than in the latter septum itself. Why, 
then, should a relatively slight difference of position, in those 
septa, be considered of such great morphological importance 
that ribs found in the horizontal septum should be called upper 
ribs, and ribs found out of it, in totally different classes or 
orders of animals, lower ribs? Goppert says (No. 48, p. 196) 
that “Gleichzeitig mit dem Auftreten der Pleuralbogen hebt 
sich das Niveau des horizontalen Septums.”’ Why not say that 
as the horizontal septum began to rise in level, the ribs, devel- 
oped primarily in connection with it, began to descend, relatively 
to it, and finally acquired a position in the wall of the peritoneal 
cavity? That in sucha process, in certain fishes, small remnants 
of the original cartilaginous “‘ Anlage” should remain attached 
to the horizontal septum, and there appear as cartilaginous parts 
of the “ Seitengraten,” or as the cartilagines intermusculares, 
would not seem improbable ; nor that, in other fishes, ribs 
should be occasionally found on the first tail vertebrae, as in 
Salmo (No. 48, p. 160). As the true upper ribs thus acquired 
a new position, the lower ribs, which Géppert considers as the 
more primitive structures, would naturally disappear. They are 
still retained in Polypterus and Calamoichthys, which thus, in 
this as in many other things, show a more primitive organi- 
zation than that of Amia or the teleosts. In selachians also, 
remnants of them seem sometimes to be still retained, as in the 

