INTEODUCTION. 
XXVll 
The latest results of Palceontology, however, do not justify other 
features in Prof. Hasse’s classification, and absolutely negative some 
of the details. 
Ssotidanus, for example, is made the type of a primitive division, 
the “ Pal®onotidani ” or “ Plagiostomi Diplospondyli,” because the 
notochord is persistent, though intercalary cartilages are developed 
in the arches ; hut, as shown in the following Catalogue, the Liassic 
llyhodux possessed a persistent notochord, and even Prof. Hasse 
himself admits that other well-known features place this genus in 
immediate proximity to Cestracion. 
Again, the Spinacidoe are regarded as representing a distinct 
division, the “ Cyclospondyli,” in which there are simple vertebral 
centra without an)’ secondary calcifications. This is obviously a 
stage in advance of the “ Palmonotidani,” being halfway between 
these and either of the two higher groups, the Tectospondyli and 
Asterosiiondyli. Put, like the Paheonotidani, the Sharks with an 
axial skeleton in this stage of development exhibit no other features 
widely separating them from the Selachians of the modern groups ; 
and it appears to the present writer that there are known cyclo- 
spondylic members of each of these groups. As already remarked, 
the semiventral position of the gill-clefts in some Spinacidae, and 
the absence of the anal fin in these fishes, are characters suggesting 
that they form the base of the Tectospondylic series. Palaeospinax 
of the Lias, with its cyclospondylic vertebrae and distinct anal fin, 
scarcely differs from the Cretaceous asterospond5’lio Synechodus, 
except in the lower degree of calcification of the vertebrae ; and the 
Lias.sic genus may therefore be placed with the Cestracionts in the 
Asterospondylic series. 
In the sum-total of characters, indeed, the degree of development 
of the vertebral centra is of small importance ; and the members of 
Haste's “ Pala;onotidani ” and “ Cyclospondyli ” may be variously 
distributed in the Tectospondyli and Asterospondyli, according as 
they approach weU-dofined types of the one group or the other. 
The acceleration of vertebral development, and the retardation of 
the same, are singular features apparently having little correspon- 
dence with the specialization or otherwise of characters still more 
likely to change. In Chlumydoselache the dentition is primitive, 
and in its close ally, ^otidanus, the teeth attain extreme specializa- 
tion ; but the former has distinct cyclospondylic vertebra;, while the 
latter only exhibits slight calcifications sometimes in the caudal 
region. The Hybodont dentition persists in the notochordal IJybo- 
