ever, stopped short in
the middle; and fretted transversely by two small but deeply-indented
rectangular marks, which, crossing from the central to two lateral
plates, assumed the semblance of connecting pins. The snout of the
Dipterus was less round; it bore no mark of the eye-orbits; and the
frontal buckler, broader in proportion to its length than that of the
Diplopterus, consisted of many more plates. I may here mention that the
frontal buckler of Diplopterus has not yet been figured nor described;
whereas that of Dipterus, though unknown as such, has been given to the
world as the occipital covering of a supposed Cephalaspian,--the
Polyphractus. Polyphractus is, however, in reality a synonym for
Dipterus,--the one name being derived from a peculiarity of the animal's
fins: the other, from the great number of its occipital plates. There is
no science founded on mere observation that can be altogether free, in
its earlier stages, from mistakes of this character,--mistakes to which
the palaeontologist, however skilful, is peculiarly liable. The teeth of
the two genera were essentially different. Those of the Dipterus,
exclusively palatal, were blunt and squat, and ranged in two
rectangular patches;[22] while those of the Diplopterus bristled along
its jaws and were slender and sharp. Their tails, too, though both
heterocercal, were diverse in their type. In each, an angular strip of
gradually-diminishing scales,--a prolongation of the scaly coat which
protected the body, and which covered here a prolongation of the
vertebral column,--ran on to the extreme termination of the upper lobe;
but there was in the Diplopterus a greatly larger development of fin on
the superior or dorsal side of the scaly strip than on that of the
Dipterus. If the caudal fin of the Osteolepis be divided longitudinally
into six equal parts, it will be found that one of these occurs on the
upper side of the vertebral prolongation, and five on the under; in the
caudal fin of the Diplopterus so divided, rather more than _two_ parts
will be found to occur on the upper side, and rather less than four on
the under; while in the caudal fin of the Dipterus the development seems
to have been restricted to the under side exclusively; at least, in none
of the many individuals which I have examined have I found any trace of
caudal rays on the upper side. These are minute and somewhat trivial
particulars; but the geologist may find them of use; and the
non-g