108 DEAN. [Vor Use 
median cusp. In no instance, is the writer aware that the 
cusps have been found worn by usage. In Cladoselache, on the 
other hand, appears a character transitional to hybodont, — its 
teeth vary in a marked manner in size and shape. Except in 
the anterior portion of the mouth, the teeth are larger on the 
inner than on the outer mouth margin, the outer teeth are 
reduced in size, lose the prominent character of median cusp 
but augment in solidity and breadth. They secure solidity by 
interwedging their broad bases, but lose thereby the ingoing 
alley-ways ; the lateral cusps function principally as neatly 
fitted buttresses to the central cusp of a succeeding neighbor. 
Even the longer cusps come in certain parts of the mouth 
to present a greatly worn appearance, suggestive of permanent 
character, as Dr. Newberry has already noted. 
CONCLUSIONS. 
Cladoselache in summary presents historic evidence as to 
the mode of origin of a number of shark structures. Its 
archaic characters, to «be expected, perhaps, from its “early 
occurrence, appear to allow no other interpretation than that 
of Wiedersheim as to the derived nature of Xenacanthid fins. 
It gives, moreover, evidence as to the antiquity of fins strikingly 
lateral-fold-like in character. As to this evidence, the con- 
servative deductions of Jaekel, even had his observations been 
justified by the material recently discovered, seem to the 
present writer exceedingly debatable. Thus, for example, his 
conclusions as to the morphological value of the embryonic 
structures of shark and Torpedo seem by no means convincing. 
Admitting that in youngest stages the fin fold of a later 
derived form, Torpedo, may present a condition more archaic, 
it does not follow necessarily that in sharks there could not 
primitively have existed a continuous fin fold. On the other 
hand, in the present stage of our knowledge of the retarding 
and acceleration of embryonic structures, Jaekel’s views would 
seem all the more questionable. Equally well might the 
living conditions of the later derived form have selectively 
evolved a latent ancestral character, which, reacquired, might 
