1872.] S0X1AS TJPPER-GREENSAND VENTRICULITES:. 63 



EXPLANATION OF PLATE VI. 



Fig. 1. A small specimen of Porospong/ia ocellata, from the Woodwardian Mu- 

 seum, 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 cyatbiform 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 

 aperturesextending to the point. This agrees with Mr. Toulmin Smith's 

 description of that part in the Ventriculidce (Ann. & Mag. Nat. Hist, 

 vol. xx. p. 91, 1847). 

 $a. 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 \ 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 tbe specimen. 



9. A portion of a slice taken from an ordinary nodule near the exterior, 



showing spicular bodies. Magnified 35 diameters. 



3. On the Ventriculit^ of the Cambridge Upper Greens.and. By 

 "W. Johnson Sollas, Esq., Associate of the Royal School of Mines, 

 London ; Scholar of St. John's College, Cambridge. 



(Communicated by the Kev. T. G-. Bonney, M.A., F.G.S.) 



[Abridged.] 



Of the sponges found in the Upper Greensand of Cambridge, cer- 

 tain forms, Seyphia, Porospcmgia, and various hitherto unnamed 

 species, are shown by this investigation to belong to the genus 

 Ventriculites. Tbe character which is stated by Toulmin Smith* to 

 be absolutely diagnostic of the Ventriculidse is the cubip arrange- 

 ment 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 wbich represent four of the six 

 rays of the hexradiate elements of wbich, on the modern view of 

 * Toulmin Smith, ' Ann. & Mag. Nat. Hist.' vol. xx. 1847, and 2nd ser. vol. i 



