Vi 
INTEODUCTIOIS'. 
ISOSPONDTLI. 
The most important Cretaceous families of primitive bony fishes 
are the Elopidse and Chirocentridne, which have a few Tertiary 
representatives, but are almost extinct at the j)resent day. Their 
distribution in the seas of the Cretaceous period is known to have 
been very wide, and individuals of the various species were 
especially abundant. 
The Elopidae, represented in the existing fauna by Flops and 
Megalops^ cannot be the direct descendants of the Jurassic Lepto- 
lepidae, because all the satisfactorily-known genera still retain the 
gular plate. This plate, however, now appears for the last time in 
the class of fishes, being completely absent in all known repre¬ 
sentatives of the higher grades; and in some of the Elopines 
themselves, such as Thrissopater (p. 33, no. 9052) and Spaniodon 
(p. 51, no. P. 9190), it is quite a rudimentary structure. The skull 
still differs very little from that of the higher Jurassic fishes, and 
the otic region remains covered by the roof-bones. In the Elopidse, 
however, the supraoccipital bone begins for the first time to exhibit 
the forward and upward growth which characterises it in all the 
higher bony fishes. The parietal bones still meet in the middle 
line in several of the genera; but even in this case the supra¬ 
occipital may extend forwards beneath them to meet the frontals 
(e. g. Megalops, p. 24). In some early genera they are distinctly 
separated in the cranial roof by the exposure of the supraoccipital. 
In this family, therefore, the latter element exhibits the usual 
inconstancy of an incipient or developing structure. 
The Albulidse are merely Elopiue fishes with a forwardly-inclined 
mandibular suspensorium, a small mouth, and reduced branchio- 
stegal apparatus. Their primitive character is, indeed, shown by 
the presence of a muscular conns arteriosus with two rows of 
valves in the heart of the sole surviving species h They seem to 
differ from the Elopidae in exactly the same manner as the more 
generalised Pycnodontidae differ from the Semionotidae among 
Jurassic fishes. Now, however, the splenial bone has disappeared 
and is no longer available to bear a powerful dentition. A new 
modification therefore occurs for the first time, and is almost con¬ 
stantly repeated in later fishes which have teeth on the palate or 
J. E. V. Boas, “ Ueber den Conns arteriosus bei Butirinus imd bei 
anderen Enochenfischen,” Morpbol, Jahrb. vol. yi. (1880), p. 528. 
