. Bulletin - United States National Museum. Science. 98 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. rounded or denticulate flange-like processes, not coming into direct contact with those of the neighboring ossicles (part 1, figs. 41, 42, 78, 85, 87, pp. 77, 131, 139, 143). When, however, the division series and arm bases are more or less turned upward they come into contact with those adjacent on either side, with the result that the possibility of lateral growth is greatly limited, and the ossicles become sharply flattened or " wall-sided " laterally but, in compensation, deep dors
. Bulletin - United States National Museum. Science. 98 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. rounded or denticulate flange-like processes, not coming into direct contact with those of the neighboring ossicles (part 1, figs. 41, 42, 78, 85, 87, pp. 77, 131, 139, 143). When, however, the division series and arm bases are more or less turned upward they come into contact with those adjacent on either side, with the result that the possibility of lateral growth is greatly limited, and the ossicles become sharply flattened or " wall-sided " laterally but, in compensation, deep dorsoven- trally (part 1, figs. 43, 44, p. 77). Ordinarily, this lateral flattening is confined to the elements proximal to the first brachial syzygy, ossicles which for phylo- genetic reasons are broader than those following, but in extreme cases it may be traced almost as far as the twentieth brachial of the free undivided arm (fig. 208, p. 146). In a few instances species with broad and patent central structures have de- veloped an excessive broadening of the division series and first two brachials, and in these cases the ossicles which have become broad have come into lateral con- tact and have developed the sharply flat- tened sides. A good example of this is seen in Comanthus solaster. Carpenter believed that the lateral flattening of the division series and lower brachials of the free arms, or "wall-sided- ness" as he called it, was, together with the commonly correlated presence of strongly developed side and covering plates, an attribute of deep water species, and that it was a systematic character of the highest significance; in fact, his ^'Basicurva" "Spinifera" and "GranuUferaP specific groups are based chiefly upon it, though the two last invariably possess in addition "plated" ambulacra. Carpenter was to a considerable extent justified in this conclusion at the time he wrote, but subsequent investigation has shown that this char
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