The most beautiful portion of the mosque, however, still exists in the prayer chamber of Hakim, where are to be found the earliest examples of the **cusped** arch and the origin of many of the geometrical patterns in stucco at the Alhambra.

Rootless, and either **cusped** or formed of parallel plates, this diversityof structure often occurring in the same family.

A more advanced phase is represented in the European Lower Oligocene by the Pseudosciuridae, with the genera Pseudosciurus, Sciuroides, Trechomys, Theridomys, &c., in which part of the masseter passes through the broad infra-orbital canal, and the premolars are; the molars being low-crowned, many-rooted and either **cusped** or ridged.

The cheek-teeth form a continuous series, gradually increasing in size from the first to the last: the molars having square four-**cusped** crowns.

Among the finest examples of this sort of work are the screens round the tombs of the Scala family at Verona, 1 35 0 - 1 375, - a sort of network of light **cusped** quatrefoils, each filled up with a small ladder (scala) in allusion to the name of the family.

The epicycloid shown is termed the "three-**cusped** epicycloid" or the "epicycloid of Cremona."

If the ratio of the radii be as I to 4, we obtain the four-**cusped** hypocycloid, which has the simple cartesian equation x 2'3+ y 213 = a 21 '.

The polar form is {(u+p) cos 26} a+{(u-p) sin 20) a = (2k)t, where p and k are the reciprocals of c and a, and u the reciprocal of the radius vector of any point on the caustic. When c =a or = oo the curve reduces to the cardioid or the two **cusped** epicycloid previously discussed.

The arches are sometimes **cusped**; circles with trefoils, quatrefoils, &c., are introduced into the tracery, and large rose windows in the transept or nave, as at Lincoln (1220).