. Bulletin. Natural history; Natuurlijke historie. 20 PEABODY MUSEUM BULLETIN 40 cap of dentine and enameloid is always smooth and flowing and that in certain cases there is the appearance of two more or less rounded patches of cosmine having fused together. Each tubercle contains a large number of pore-cavities and dentinal units; even the smallest "seedlike" patches of cosmine contain several units of both systems. The histological structure of these tubercles and their significance are considered further below. The distribution of the tubercles on the dermal bones of the head is m
. Bulletin. Natural history; Natuurlijke historie. 20 PEABODY MUSEUM BULLETIN 40 cap of dentine and enameloid is always smooth and flowing and that in certain cases there is the appearance of two more or less rounded patches of cosmine having fused together. Each tubercle contains a large number of pore-cavities and dentinal units; even the smallest "seedlike" patches of cosmine contain several units of both systems. The histological structure of these tubercles and their significance are considered further below. The distribution of the tubercles on the dermal bones of the head is most interesting (Figs. 5,6, 7, 8, 29, 30, 31 and 34). They are never found on the ventral elements—the mandibles and gular series. They are not found on the very tip of the snout. In these regions, as indicated above, there is normally a more or less complete cover of cosmine. The tubercles are distributed only on the upper parts of the head and are found there on all elements. On the skull table (Fig. 8) they are arranged in two rows on each side of the head. One row passes along the tabular, supratemporal, intertemporal, parietal and rostral bones in a course exactly coincident with that of the principal lateral line of the head. The inner row passes along the postparietal bone to the parietal. In addition, there are usually one large tubercle and also frequently several smaller tubercles on the large medial postrostral bone. On each bone the tubercles are arranged near the center of growth of the bone, the only exception being the very long postparietal element. In most cases the tubercles are found in a radiating pattern roughly following the series of ridges in the spongiosa surface that mark the position of the growth center of the element. Despite the focusing of the tubercle distribution over the growth centers, they nonetheless form distinct rows and one may suggest that the inner row, described above, "joins" the outer row where the latter moves over on to
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