Dr. H. Rauff ow the Genus Hindia, Dune. 171 



In what follows I shall not give the details and the full 

 account of all mj observations, which would be impracticable 

 without fig-ures from the microscopic objects, and must remain 

 over for a larger memoir ; with regard to the form, the texture 

 of the surface, the structure of the fractured surfaces with 

 their radial streaks, and many details of the microscopic ap- 

 pearances we can refer to Duncan's excellent description. The 

 additions and corrections, chiefly relating to Duncan's micro- 

 scopic observations, which I have now been enabled to make 

 by the more abundant material at my disposal will appear of 

 themselves upon a comparison of what follows with Duncan's 

 memoir ; I would here only communicate the essentially new 

 results relating to the wonderful, perhaps unprecedentedly 

 regular and elegant construction of the sponge-skeleton from 

 the individual elements as it appears to me from the micro- 

 scopic preparations, and which perhaps may serve to give 

 these bodies, so insignificant in external appearance, quite a 

 peculiar interest. 



From the central point of the spheres * — it must remain a 

 question whether around this there was a small cavity or a 

 foreign body (Hinde), or whether the tissue originally pos- 

 sessed a somewhat different and looser structure (Duncan) — 

 there radiate in all directions thin, perfectly straight canals, 

 which differ somewhat from each other in diameter, are slightly 

 widened externally, and are increased in number towards the 

 surface by the interpolation of new ones ; these present a more 

 or less regular transverse section, and their perforated walls 

 are formed by quadriradiate, tetracladine skeletal elements. 



The typical tetracladine element consists of four arms radi- 

 ating from a point, and arranged in space in the same way as 

 the Imes running from the centre of gravity of a tetrahedron 

 to its four angles, or, which of course is the same thing, the 

 perpendiculars from this centre of gravity to the surfaces of 

 the 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 projection 

 perpendicular to the fourth (that is to say, therefore, the pro- 

 jections of three rays upon the surface of the tetrahedron 

 defined by the apices of these rays) enclose 120" between each 

 two rays. 



The elements of Hindia appear to be all very similar. In 

 radial sections the microscope usually enables us to recognize 

 only two arms distinctly, as the elements are rather large, and 

 the thin sections, to furnish distinct images, must be kept very 

 thin in proportion to this size ; of the third arm only the broken 



* I have them before me of 10-45 millim. in diameter. 



12* 



