FISHES OF THE CARBONIFEROUS SYSTEM. 993, 
The segmented structure of Edestus is its most marked and anomalous 
feature, but one equally so whether it be considered spine, jaw, rostrum, or 
intermandibular arch. It is undoubtedly to this structure that we must 
ascribe the absence of a medullary cavity, as each segment bearing a 
denticle seems to have been nourished independently of its fellows. It is 
also apparent that the growth of this organ was by additions to the summit 
of successive sheaths, each of which carried a denticle. ‘This is strikingly 
different from the mode of growth of all sharks’ spines known, as these in- 
crease by additions to the base, and are thus pushed upward and lengthened. 
The same is true of all rostra which are used as weapons of defense or 
offense. If we consider the segments of Edestus as homologues of a dental 
series we encounter the same difficulty. A row of teeth of Orodus, for ex- 
ample, which consist of enameled crowns with flattened bony bases lying 
in contact and compressed together, considered as a whole, presents some 
analogy with our fossils; but there, too, the growth is from behind forward, 
new teeth moving up to take the places of such as are broken or worn away. 
The numerous disconnected segments of Hdestus Heinrichst furnished me 
by Mr. Butters seem to prove conclusively that the spine was elongated by 
the addition of a sheath carrying a denticle to the extremity and under 
side of the pre-existing series, as shown in PI. KOCK Fig. 2", dalso 
have from Vermillion County, Ind., a specimen figured on Bis XOXO 
Fig. 2°, which seems to be the basal segment of a spine, probably of a 
young individual of Edestus Heinrichsi. ‘This isa spatulate solid bone, car- 
rying a beautifully perfect enameled denticle at its extremity. The shells 
or sheaths obtained from Mr. Butters are similar to this, except that each 
one is a trough, into which the succeeding one fits, and the added cap cov- 
ers a portion of the enameled base of its predecessor. If this is all true, 
and it seems undeniable, we are compelled to conclude that the spine was 
buried in the integuments throughout its entire length, the enameled denticles 
alone projecting above the surface to form a saw, which would be a terrible 
weapon if placed upon some flexible portion of the body where it could be 
used with freedom and power. The extremity of the spine may have lain 
in a sheath, from which it could be partially erected by muscular action and 
used as the lancet of the surgeon fish (Acanthurus) is; but the bilateral 
