move was to construct the portico outside the slab.  The portico then needed a roof, and the addition of a second cover to provide it completed the transition to the simpler corridor-tomb.  In many cases the Irish simple dolmens were surrounded by a circle of upright stones.  At Carrowmore, Sligo, there seems to have been a veritable cemetery of dolmen-tombs, each of which has one or more circles around it, the outermost being 120 feet in diameter.  The tombs in these Carrowmore circles were not always simple dolmens, but often corridor-tombs of more or less complicated types.  Their excavation has not given very definite results.  In many cases human bones have been found in considerable quantities, sometimes in a calcined condition; but there is no real evidence to show that cremation was the burial rite practised.  The calcination of human bones may well have been caused by the lighting of fires in the tomb, either at some funeral ceremony, or in even later days, when the place was used as a shelter for peasants.  A few poor flints were found and a little pottery, together with many bones of animals and some pins and borers of bone.  The most important find made, however, was a small conical button made of bone with two holes pierced in its flat side and meeting in the middle.  It is a type which occurs in Europe only at the period of transition from the age of stone to that of bronze, and usually in connection with megalithic monuments.

[Illustration:  FIG. 6.  Type-plan of the simple rectangular corridor-tomb
               or allee couverte.]

We pass on now to consider the simplest form of corridor-tomb, that in which there are several cover-slabs, but no separate chamber (Fig. 6).  These tombs occur in most parts of Ireland.  At Carrick-a-Dhirra, County Waterford, there is a perfect example of the most simple type.  The tomb is exactly rectangular and lies east and west, with a length of 19 feet and a breadth of 7-1/2.  At each end is a single upright, and each long side consists of seven.  The chamber thus formed is roofed by five slabs.  The whole was surrounded by a circle of about twenty-six stones, and no doubt the chamber was originally covered by a mound.  In a somewhat similar example at Coolback, Fermanagh, the remains of the elliptical cairn are still visible.

But in most cases the plan of the corridor-tomb is complicated by a kind of outer lining of blocks which was added to it.  Most of the monuments are so damaged that it is difficult to see what the exact form of this lining was.  Whether it merely consisted of a line of upright blocks close around the sides of the chamber or whether these supported some further structure which covered up the whole chamber it is difficult to say.  In some cases the roof-slab actually covers the outer line of blocks, and here it seems certain that this outer line served simply to reinforce the chamber walls, the space between being filled with earth or rubble.  However, at