. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. 94 THREE CRUISES OF THE " ; The oldest known sea-urchins belong to the Palaechinidae, a group of palaeozoic echini, having, unlike their modern conge- ners, more than two rows of plates in each zone of the test, and with plates overlapping like the tiles of a roof, so that the test must have possessed considerable flexibility. These urchins were succeeded in mesozoic times by types with a still more flexi- ble test, the coronal plates forming a continuous series from the mouth to the apical sy
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. 94 THREE CRUISES OF THE " ; The oldest known sea-urchins belong to the Palaechinidae, a group of palaeozoic echini, having, unlike their modern conge- ners, more than two rows of plates in each zone of the test, and with plates overlapping like the tiles of a roof, so that the test must have possessed considerable flexibility. These urchins were succeeded in mesozoic times by types with a still more flexi- ble test, the coronal plates forming a continuous series from the mouth to the apical system without the usual sharp distinctions of aetinal, coronal, and apical systems. This group is repre- sented in our seas by the Echinothuriae. We may call attention to the characteristic genus Asthenosoma, belonging to the type of echini with flexible test and overlapping plates (Fig. 359 a), first described by Grube from a single specimen, and subsequently collected by the " Chal- Fig. 359a.—Asthenosoma hystrix. , ,, ~ , ,. , lenger. brube did not, however, recognize the great importance of his discovery, and it was not until Thomson and Pourtales dredg-ed these flexible urchins that their affinity to the Echinothuriae of the chalk and to the Palaechinidae became evident. Traces of this overlapping of the coronal plates can still be detected in the most specialized of the recent sea-urchins. In one of the hauls taken between Cape Maysi and Jamaica (1,200 fathoms), we obtained the first specimens of Asthenosoma (Fig. 359) I had seen alive. I was much astonished to find them, fully blown up, hemispherical or globular in shape. This was the shape they always took in subsequent hauls, and on several occasions, when they were obtained from comparatively shallow water near the 100-fathom line, they came up alive, and retained their globular outline. The alcoholic specimens I had seen in the "Challenger" collection dredged from deep water were as flat as pocket-han
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