/^3>3 r /^c. 
IXTRODUCTIOX. xxi 
Oligopleuridas with the latter family at the base of the Isospondylic 
series. 
Leptolepidce. 
The Leptolepidse differ from the two preceding families in the 
absence of fulcra on the fins, and are remarkable as being the 
earliest family in which intermuscular bones occur. These 
elements, forming so conspicuous a feature among modern fishes, 
appear to he arranged here only in a single series above the vertebral 
column in the abdominal region ; though there may perhaps be 
traces of them sometimes also in the lower half of the caudal 
region. The vertebral centra in Leptolems itself exhibit interesting 
gradations in the degree of development according to the geological 
age of the species; these centra never being more than delicate 
constricted rings or cylinders in the Upper Lias, and always 
strengthened by secondary peripheral calcifications in the Oxfordian 
and upwards. In Thrissops, which ranges as far at least as the 
Lower Cretaceous, the vertebral centra are still more robust. 
These fishes, it will be noticed, approach very closely the 
Clupeidas, among which they are sometimes included; but they 
are distinguished by the meeting of the parietal bones in the 
median line, by the non-fusion of the haemal spines at the base of 
the tail, and by the presence of a thin film of ganoine on the 
scales. 
The interpretation of the relationships of the Mesozoic Actino- 
pterygian fishes thus briefly summarized, differs so much from any 
hitherto proposed that it has not been found possible to arrange a 
synonymy under the subordinal and family-headings in the 
Catalogue. Uamily-names not derived from generic names have 
also been ignored. The existing Acipenser , Polyodon, Spcitidaria, 
Lepidosteus, and Amia are now shown to afford a very inadequate 
and misleading idea of the actinopterygian ganoids, on account of 
their remarkably specialized nature. The characters of the appen¬ 
dicular skeleton are observed to be much more important than those 
even of the axial skeleton of the trunk, while the form and absence 
or presence of the scales, as also the degree of development of the 
branchiostegal apparatus, are features comparatively insignificant. 
It is thus no longer scientific to regard the “ Acipenseroidei ” as 
typical members of the group to which they belong ; they are mere 
