1872.] SOLLAS-UPPER-GREENSAND VENTRICULITAE. 63
EXPLANATION OF PLATE VI.
Fig. 1. A small specimen of Porospongia ocellata, from the Woodwardian Museum, Cambridge.
2. Half a similar specimen. These two specimens show no apertures on the opposite surfaces.
3. Quadrate reticular structure as seen in a horizontal section of part of the removed portion of the specimen fig. 2. The area figured has the reticulation less regularly quadrate than usual. Magnified 60 diameters.
4. A view of a node of the preceding, with the two adjoining nodes less distinctly displayed on account of the obliquity of the plane of section. Magnified 60 diameters.
5. A view of one out of several such bodies which occur in the slice from which figs. 3 and 4 are taken. They appear to have been spherical. Are they reproductive germs? Magnified 250 diameters.
6. A cyathiform specimen having smaller apertures. The regularity of the form is obscured by the adhesion of a mass of phosphate.
The lower extremity is imperforate, and tapers off regularly, the apertures extending to the point. This agrees with Mr. Toulmin Smith's description of that part in the Ventriculidoe (Ann. & Mag. Nat. Hist. vol. xx. p. 91, 1847).
6a. The upper extremity of the same specimen.
7. Quadrate reticular structure as seen in a transverse slice of a specimen of the same kind as the last. The plane of section passes through a plane of nodes at the bottom of the figure, but avoids them in the upper part, there cutting only the intermediate canals. Magnified 25 diameters.
The slice, of which a part is figured, was taken at about 1/2 an inch from the upper extremity of a specimen of the same size as fig. 6, and proves the central cavity to have been narrow. But its condition is unfavourable for taking an exact measurement.
8. External aspect of an ordinary nodule, showing the wrinkled surface and some shrinkage-cracks. A surface of attachment is on the further side of the specimen.
9. A portion of a slice taken from an ordinary nodule near the exterior, showing spicular bodies. Magnified 35 diameters.
3. On the Ventriculitae of the Cambridge Upper Greensand. By W. Johnson Sollas, Esq., Associate of the Royal School of Mines, London; Scholar of St. John's College, Cambridge.
(Communicated by the Rev. T. G. Bonney, M.A., F.G.S.)
[Abridged.]
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 Ventriculidae 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
- Toulmin Smith, 'Ann. & Mag. Nat. Hist.' vol. xx. 1847, and 2nd ser. vol. i.
