Umbilical vesicle, [82].

Ungulata, [25], [109].

Ungulata eocene, [110].

Units, physiological, [168], [218].

Unknowable, the, [245].

Upper Silurian strata, [140], [142].

Urotrichus, [68].

V.

Variability, different degrees of, [119].

Vermiform appendix, [83].