. A contribution to American thalassography : Three cruises of the United States Coast and geodetic survey steamer "Blake", in the gulf of Mexico, in the Caribbean sea, and along the Atlantic coast of the United States, from 1877 to 1880. Blake (Steamer); Marine animals -- Atlantic Ocean; Marine sediments. 94 THREE CRUISES OF THE " ;. Fig-. 350 a. — Asthenosoma hystrix. The oldest known sea-urchins belong to the Palsechinidije, 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
. A contribution to American thalassography : Three cruises of the United States Coast and geodetic survey steamer "Blake", in the gulf of Mexico, in the Caribbean sea, and along the Atlantic coast of the United States, from 1877 to 1880. Blake (Steamer); Marine animals -- Atlantic Ocean; Marine sediments. 94 THREE CRUISES OF THE " ;. Fig-. 350 a. — Asthenosoma hystrix. The oldest known sea-urchins belong to the Palsechinidije, 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 actinal, coronal, and apical systems. This group is repre- sented in our seas by the Echinothurise. We may call attention to the characteristic genus Asthenosoma, belonging to the type of echini with flexible test and overlapping plates (Fig. 359 «), first described by Grube from a single specimen, and subsequently collected by the " Chal- ; Grube did not, however, recognize the great importance of his discovery, and it was not until Thomson and Pom-tales dredged these flexible urchins that their affinity to the Echinothuriee of the chalk and to the Palaechinidse 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 compara
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Keywords: ., bookauthoruscoasta, bookcentury1800, bookdecade1880, bookyear1888
