XXII 
INTRODUCTION. 
paired fins are truly archipterygial, whether elongate or abbreviate; 
while in the existing Polypteridas the pectoral fins have lost all 
trace of the original branched arrangement of the cartilages (pre¬ 
cisely like the Sharks), and in Polypterus itself the pelvic fins are 
approximately in the same condition as those of one of the Actino- 
pterygian Chondrostei. Among the early families, the characters 
of the median fins lead to the recognition of two or three divisions. 
It is probable that one type in which the median fin remains 
undivided and more or less in its primitive condition will eventually 
be met with, even if it be not already known. This group has 
received the name of Haplistia, and we provisionally assign to it 
the problematical Tarrasikke. The second and third types, though 
now clearly definable, are not satisfactorily formulated in the some¬ 
what fluctuating classifications of Cope; and the terras Bhipidistia 
and Actinistia are selected on the present occasion from those 
already proposed by that author, as being most expressive and 
accurate. For their diagnosis and description, reference may be 
made to the Catalogue itself; and it only seems necessary here to 
remark upon the extraordinary manner in which the specialized 
dorsal fins of the Bhipidistia resemble paired limbs (see especially 
fig. 50, p. 335). When subdivided, the dorsal fin invariably 
degenerates to two portions, and these are supported on a plan that 
does not differ much from that of an abbreviate archipterygium. 
The great group of Actinopterygian Teleostomi is that con¬ 
cerning which palaeontology affords most extensive information ; 
but as only the typically Palaeozoic families of Palaeoniscidae and 
Platysomatidae are comprised in the present volume, it will be con¬ 
venient to defer general observations on their relationships until the 
completion of Part III. 
In conclusion, there is little to add concerning the details of the 
plan of the Catalogue to the statement already made in the Intro¬ 
duction to Part I. Family names derived from generic names 
terminating in -aspis and -lejpis occur now for the first time; and, 
from the point of view of euphonj^, it has been deemed advisable 
to omit the reduplication of “ id” which would be necessitated by a 
strict adherence to classical rule. There is already justification for 
this procedure in the universal adoption of the term Crossopterygii 
instead of the strictly accurate Crossotopterygii. More descriptive 
sections have been included than in the former volume, on account 
of the importance of the Palaeozoic types to the modern Biologist, 
