attacked the problem of resolving the higher algebraic equations,
notwithstanding Abel's proof that a solution by radicles was impossible.
In this field Cockle achieved some notable results, amongst which is his
reproduction of Sir William R. Hamilton's modification of Abel's
theorem. Algebraic forms were a favourite object of his studies, and he
discovered and developed the theory of criticoids, or differential
invariants; he also made contributions to the theory of differential
equations. He displayed a keen interest in scientific societies. From
1863 to 1879 he was president of the Queensland Philosophical Society
(now incorporated in the Royal Society of Queensland); on his return to
England he became associated with the London Mathematical Society, of
which he was president from 1886 to 1888, and the Royal Astronomical
Society, serving as a member of the council from 1888 to 1892. He died
in London on the 27th of January 1895.

A volume containing his scientific and mathematical researches made
during the years 1864-1877 was presented to the British Museum in 1897
by his widow. See the obituary notice by the Rev. R. Harley in _Proc.
Roy. Soc._ vol. 59.

COCKLE, in zoology, a mollusc (_Cardium_) of the class Lamellibranchia
(q.v.). A very large number of species of _Cardium_ have been
distinguished by conchologists. Besides the common species _Cardium
edule_, two others occur in Britain, but are not sufficiently common to
be of commercial importance. One of these is _C. echinatum_, which is
larger than the common species, reaching 3 in. in diameter, and
distinguished by the presence of spines along the ribs of the shell. The
other is _C. norvegicum_, which is also somewhat larger than _C. edule_,
is longer dorso-ventrally than broad, and is only faintly ribbed.

The two valves of the shell of the common cockle are similar to each
other, and somewhat circular in outline. The beak or umbo of each valve
is prominent and rounded, and a number of sharp ridges and furrows
radiate from the apex to the free edge of the shell, which is crenated.
The ligament is external, and the hinge carries cardinal teeth in each
valve. The interior of the shell is remarkable for the absence of pearly
lustre on its interior surface. The colour externally is reddish or
yellowish. The pallial line, which is the line of attachment of the
mantle parallel to the edge of the shell, is not indented by a sinus at
the posterior end