96 General Notes. [January, 
boganoidei, Cycloganoidei, Crossopterygia and Chondrostei show 
a more decided tendency towards the development of a dorsal and 
ventral, or, only a ventral series of caudal rays which extend to 
the end of the caudal axis, and thus trend more towards a diphy- 
cercal condition than the existing Teleostei, which may be said to 
be verging towards hypocercy when all of the caudal rays will be 
of hypaxial origin, with very often a rayless interval between the 
last hypaxial pieces and the end of the exserted urostyle (Fig. 8), 
the latter finally tending to become shorter and be aborted as in 
: Fistularia and Apeltes. These are some of the marks of pro- 
gress which distinguish the Teleosts and supplement the signifi- 
cant fact of their well-ossified skeleton. It is highly probable 
that we shall find no remains of the larvz of Paleozoic fishes in 
the rocks, so that we have no means of contrasting their early 
phases with those of existing forms, but it is certain that 
none of the most simple forms of the Palzozoic fishes, in respect 
to their caudal skeletal structure, even approximate such a primi- 
tive condition as the lophocercal stage of modern forms; the 
only trait which they possess in common are the continuous me- 
dian fins ; in the first instance containing rays, in the latter case 
being without them. When we know the larve of Ceratodus, 
Polypterus, Lepidosiren and Protopterus, as well as we know that 
of Lepidosteus we may have a moderately comprehensive under- 
standing of the main features of the development of Palzeozoic 
hes. 
The evidence in favor of degeneration of portions of the cau- 
dal region of fishes is the existence of a permanent archicercal 
opisthure in Chimera monstrosa and Stylephorus chordatus ; the 
extensive development of a temporary opisthure in Lepidosteus ; 
the concrescence of the hypural pieces; the ventrally diplacan- 
thous and even triplacanthous caudal vertebre (Fig. 8), or their 
coalesced representative, the urostyle ; the existence of hypaxial 
opisthurai elements; the abortion of the epaxial spines of the 
caudal vertebra, and finally the abortion or extreme modification 
of the last muscular somites of the caudal region. 
5. Homocercy—This merely expresses the condition of epaxial 
and hypaxial symmetry presented by the fan-shaped caudal of 
Teleosts, and is the final term in the evolution of the growth of 
the rays of that fin, in consequence of which the archaic symme- 
try of perfect diphycercy becomes again restored, though the 
axial structure of the tail is heterocercal. 
6. Gephyrocercy.—This type of tail appears to be normally met 
with in only two forms of Teleosts, viz., Mola and Fierasfer. 
The primitive opisthure or end of the urosome in these forms is 
apparently aborted, in the first, in the course of larval existence, 
in the other during post-larval life. As a result of this a hiatus 
is left between the epaxial and hypaxial rudiments of the median 
fins, and in the center of this hiatus the axial column ends 
