On the Ventriculitæ of the Cambridge Upper Greensand

Abstract

Of the sponges found in the Upper Greensand of Cambridge, certain forms, Scyphia, Porospongia , and various hitherto unnamed species, are shown by this investigation to belong to the genus Ventriculites . The character which is stated by Toulmin Smith to be absolutely diagnostic of the Ventriculidæ is the cubic arrangement of the fibres supported at the angles by octahedral stays; and on making sections of the above sponges this structure may at once be distinguished under the microscope. It appears in the most favourable sections as a square or rectangular reticulation, composed of a number of crosses, the arms of which represent four of the six rays of the hexradiate elements of which, on the modern view of Ventriculite-structure, the skeleton is considered to be composed. On examining one of the above crosses, it is seen that its radii start from their common centre as very fine filaments indeed; but after pursuing their course for a quarter of their entire length, they undergo a sudden enlargement in diameter, and continue uniformly of this increased size to their terminal anastomosis. At the point of rather abrupt change in the diameter of the radius two other fibres are given off, one on each side, which pass backwards at an angle of about 45°, to join similar fibres similarly passing from the other arms of the cross; in this manner, about the centre of the cross under consideration, is formed a rough square, which represents the octahedral stays in section (fig. 1). This description