FOSSIL SHARKS 77 
VIII. Dicestive TuBE with a single bend, S, J, the 
intestine provided with a spiral valve (p. 263), terminat- 
ing, together with the ducts of the renal and reproduc- 
tive organs, in a common cloaca, CL (p. 266). Liver, Z, 
spleen, and pancreas large ; mesenteries simple but greatly 
fenestrated ; air bladder absent. 
IX. Heart with a contractile arterial cone, CA, con-- 
taining several rows of valves (p. 260) ; circulatory system 
in general as described on p. 269. 
X. “Caspers” developed at the hinder margin of 
the ventral fins as the intromittent organ of the male. 
They are rudimentary in the female, CZ’. Each clasper 
is the trough-like hinder rim of the fin, which becomes 
transformed into the compact, elongated, tube-like sperm 
canal. Its tip is often studded with elongated shagreen 
denticles whose recurved cusps retain it 2 copulo. 
Fossil Sharks 
Of all fishes, sharks certainly suggest most closely in 
their general structures the metameral conditions of the 
Cyclostome: it should also be noted that they possess the 
greatest number of body segments, in some instances 
over three hundred, known among vertebrates. Little is 
known, however, of the primitive stem of the sharks, and 
even the lines of descent of the different members of the 
group can only be generally suggested. The development 
of the recent forms has yielded few results of undoubted 
value to the phylogenist: it would appear as if palaon- 
tology alone could solve the puzzles of their descent. 
The history of fossil sharks has as yet been but imper- 
fectly outlined. The remains of the more ancient forms 
have usually proven so imperfectly preserved that little 
could be determined of their structural characters. Spines, 
