Skcondary Forms, 485 conceivable shape and size ; but the cut is sometimes sufficiently irregular to make it difficult to define, in the vertical plane, the section described by the arrangement of the courses ; hence it is not always easy to make them out in the structure. Some- times the uppermost stone really acts as the key-stone of an arch ; whence one is almost tempted to believe that the principle of the vault has been grasped (Fig. 7^). Generally, however, we find two terminal blocks, which either lean directly against ^ as > Fig. 193.— Entrance lo gallery, easl wall. each other at the upper edges, or against a third stone set between them from above (Figs. 74, 78, 193); or, again, the unit is horizontally placed in lintel-wise fashion. A very similar arrangement is beheld in a circuit of Samothrace. The island was known to have been peopled by Pelasgi, and their dialect was spoken there in historical times. It is the same with the small entrance, triangular in section, pierced in the citadel wall of Mycense (Fig. 95), to provide the garrison with a secret means of access to the Perseia. This opening, with its sloping sides,
