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Show 280 DR. BOWERBANK ON SILICEO-FIBROUS SPONGES. [Apr. 6, liarities of the skeleton ; and in these fortunately there are very sufficient distinctive characters to assist us in the description of the species. In some of the little patches of this parasitical sponge there are several layers of the skeleton-structure, while in other cases it spreads like a single network over the surface of the sponge upon which it is parasitical. There is no definite arrangement in the skeleton-rete, and tbe areas assume a great variety of forms. The skeleton-fibre is always more or less depressed ; and in some cases two or more fibres coalesce, forming small broad plates of siliceous structure in which two or three canals m a y be seen running in parallel lines. The fibres in their normal condition vary to a considerable extent in their diameter : the largest measured was T*-T inch, whilst the smallest was -^ohis ^ncn in diameter ; the average diameter of four measured was } 3l ^ inch. The central canals of the fibres also vary in their diameter; the largest was -31 \ 6 inch, and the smallest sijW iac^ iQ diameter. Their course through the fibres is not always continuous; and frequent cases occur in which they terminate abruptly before reaching the angle of the network towards which they are progressing. The specimen described is the only one I have yet seen; but it is probable that it will be found parasitical on other species of siliceo-fibrous sponges beside Farrea gassioti. The locality is the same as that of F. gassioti, lat. 14° 8' N., long. 77° 38' W., in 800 to 1000 fathoms. * EXPLANATION OF THE PLATES. PLATE XXXLX. Fig. 1. Farrea gassioti, natural size. 2. A small portion of the outer surface of the skeleton of F. gassioti from the distal margin of the specimen represented by fig. 1, x 3 6 linear. 3. A small portion of the piece of the skeleton represented by fig. 2, exhibiting the mode of the canaliculation of the fibre of the skeleton, X 80 linear. 4 & 5. T w o specimens of Farrea pocillum, natural size. 6. A portion of the skeleton of F. pocillum from the specimen represented by fig. 5, exhibiting the more or less irregular mode of its reticulation, X 36 linear. 7. A small piece of the skeleton of the specimen represented by fig. 4, exhibiting the spination of the skeleton-fibres and the numerous rectangulated sexradiate defensive organs in situ, X 80 linear. 8. One of the rectangulated sexradiate defensive organs, exhibiting the mode of its spination, x 150 linear. PLATE XL. Fig. 1 represents a specimen of Deanea. virgultosa, natural size. 2. A portion of the skeleton of the specimen represented by fig. 1, exhibiting the rotulate mode of arrangement of the skeleton-rete and the large central canals within the fibre, X 36 linear. 3. Farrea fistulata, natural size. 4. A small piece of the skeleton of the specimen represented by fig. 3, exhibiting the quadrangular form of the rete and their large'central canals, X 36 linear. 5. A portion of a specimen of Farrea Icevis, natural size. 6. A fragment of the specimen represented by fig. 5, exhibiting the form of |
