. The Annals and magazine of natural history; zoology, botany, and geology. ame thing, theperpendiculars from this centre of gravity to the surfaces ofthe tetrahedron. The four arras are therefore equally distri-buted in space, and meet each other at an angle of 109° 28;but the projections of each three rays in a plane of projectionperpendicular to the fourth (that is to say, therefore, the pro-jections of three rays upon the surface of the tetrahedrondefined by the apices of these rays) enclose 120 between eachtwo rays. The elements of Hindia appear to be all very similar. Inradial sections t
. The Annals and magazine of natural history; zoology, botany, and geology. ame thing, theperpendiculars from this centre of gravity to the surfaces ofthe tetrahedron. The four arras are therefore equally distri-buted in space, and meet each other at an angle of 109° 28;but the projections of each three rays in a plane of projectionperpendicular to the fourth (that is to say, therefore, the pro-jections of three rays upon the surface of the tetrahedrondefined by the apices of these rays) enclose 120 between eachtwo rays. The elements of Hindia appear to be all very similar. Inradial sections the microscope usually enables us to recognizeonly two arms distinctly, as the elements are rather large, andthe thin sections, to furnish distinct images, must be kept verythin in proportion to this size ; of the third arm only the broken * I have them before me of 10-45 millim. in diameter. 12* 172 Dr. H. RaufF on the Genus Hindia^ Dune. or amputated cylindrical stump is then recognizaLle, wliile thefourth is very seldom observed, and, in fact, appears generally Fi- to be rudimentary or entirely deficient. It may be remarked,however, that it has several times been recognized withcertainty. The two distinct rays present a very characteristic form ;they are curved in the same direction, the concave marginalways appears smooth, the convex one always toothed, andeach of the extremities of the arms (heads), which are widenedin a direction about perpendicular to the arm, and lie in thesame surface of the canal-wall with the arm, are also toothedon the outer margin. As all the radial sections appear to beequivalent, i. e. furnish tlie same images, we are justified inassuming that the third arm also has the same structure as thetwo just mentioned, and that the diagrammatic representationin fig. 1 (the shaded part) may represent the Hindia ele-ment *. These individual elements are united to one another in aremarkable manner. When the connexion of the skeleton ispreserved in tlie p
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