ss that they have
grown so large that the passage will not admit them. The following are
among the concretions found in the various parts:

(1) _Coralline calculi._--These are of a dull-white color and irregular
surface, like coral. They are made up of hard and resistant layers evenly
deposited around a central nucleus. (Pl. XI, fig. 3.) Their specific
gravity is 1,760, water being 1,000, and they contain 74 per cent of
carbonate of lime with some carbonate of magnesia, organic matter, and a
trace of carbonate of iron. Yellowish-white, smooth, round calculi of the
same chemical composition are met with.

(2) _Pearly calculi._--These are more frequent than the first-named
variety. They are very hard and smooth on the surface, reflecting a play of
various colors after the fashion of a pearl. This peculiarity appears to be
caused by the thinness and semitransparency of the supposed layers. They
have a specific gravity of 2,109 to 2,351, and nearly the same chemical
composition as the coralline variety. Golding Bird found a specimen of this
kind formed of carbonate of lime and organic matter only.

(3) _Green calculi_ (_metalloid calculi_).--These are usually small and
numerous, as they are exceedingly common. They are of a very hard
consistency, and have a clear-polished, greenish surface of almost metallic
brilliancy. They have a specific gravity of 2.301 and a composition almost
identical with the second variety.

(4) _White calculi._--Pure white, smooth, lustrous calculi are rare. They
have a specific gravity of 2.307, and contain as much as 92 per cent of
carbonate of lime with carbonate of magnesia and organic matter.

(5) _Ammonio-magnesium calculi._--These are of a grayish color and a very
rough, crystalline surface, which proves very irritating to the mucous
membrane. They have a specific gravity of 1.109 to 1.637, and are composed
chiefly of ammonio-magnesium phosphate, oxalate of lime, and organic
matter, with a little carbonate of lime and magnesia.

(6) _Siliceous calculi._--These are clear, smooth, and hard, and usually
spherical. They have a specific gravity of 1.265 to 1.376, and contain 57
per cent of silica with carbonates of iron and magnesia, organic matter,
and traces of iron. In other specimens of siliceous calculi there was a
specific gravity of 3.122, and there was 79 to 85 per cent of carbonate of
lime together with carbonate of magnesia, and iron, silica, and organic
matter. Others are almost