Xll 
INTRODUCTION. 
centra are never fully formed, and they only reach even the annular 
stage when, notwithstanding specialization, the fish retains its 
original thick and continuous squamation. The cheek-plates are 
apparently never lost, but become irregularly subdivided in the 
later forms ; and there is always a tendency to reduction of the 
branchiostegal apparatus. The scales often degenerate or disappear 
in certain parts—on the back in Macrosemius , on the tail in Aphne- 
lejris, Aetheolepis , and Tetragonolepis. Fulcra are least conspicuous 
in the genera which exhibit most tendency to such degeneration of 
the scales. 
These considerations are important because, if truly expressions 
of fact, they seem to determine the systematic position and affinities 
of the anomalous family of Pycnodontidae. As amply demonstrated 
in the Catalogue, these fishes exhibit an appendicular skeleton quite 
distinct from that of the Chondrostei, but identical with that of 
the Protospondyli and Aetheospondyli, and only distinguished from 
that of the Isospondyli by the large number of basals in the pectoral 
fin. The additional characters of the axial skeleton of the trunk 
and the mandible place them unhesitatingly among the Proto¬ 
spondyli. 
In five respects they are merely extreme members of the modified 
series of deep-bodied Protospondyli, which begins with Dapeclius. 
There is never any approach to the development of vertebral centra; 
and even when the axis acquires special rigidity (e. g. Palceoba- 
listum and Pycnodus ) this is attained by the interlocking of laminar 
expansions of the neural and haemal arches. The cheek-plates, so 
far as known, are reduced to tesserae. The branchiostegal rays are 
not more than two in number. The fin-fulcra are still more 
insignificant than in Tetragonal epis. The caudal region is very 
commonly destitute, or nearly destitute, of scales behind a line con¬ 
necting the origin of the dorsal with that of the anal fin. 
When examined more closely, it is not difficult to perceive that 
even the apparently unique characters of the Pycnodonts are also 
the ultimate result of tendencies to specialization already exhibited 
by the Semionotidse. 
Pirstly, there is the forwardly-displaced mouth with crushing- 
teeth, which is much like that of the latest species of Lepidotus , 
only a little more advanced and powerful, and thus more firmly 
fixed to the skull. The mode of fixation, however, resembles that 
observed in fishes so remotely related to each other as Gymnarchus, 
Tetrodon, and Mormyrus : proving that it is merely a physiological 
