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Show 540 DR. B O W E R B A N K O N SILICEO-FIBROUS SPONGES. [June 20, the intermediate portions between the inhalant and exhalant are so completely intermingled that it cannot be determined with certainty how many of them there are. The fibres also of which they are composed are very variable ; some of them, and especially the largest ones, are nearly quite destitute of minute spinules, while others of small diameter abound in them in an incipient state of development. The central canals of the fibres are also very irregularly developed : in some parts they are obsolete; in others they are visible, but are of extreme tenuity ; they rarely appear continuous for any distance; and frequently they terminate abruptly. The same irregularity is apparent with the interstitial defensive organs. A few single slender small attenuated spines are observable on the fibres of the skeleton, disposed at considerable distances from each other ; they are very variable in size and mode of disposition. A few rectangulated sexradiate defensive organs are also present at considerable intervals. They are mostly very minute, requiring a power of about 200 linear to render them distinctly to the eye; and if of larger dimensions, they are often abnormal and distorted in form. The prevailing irregularities in the structure of this species, in addition to the more positive specific characters, will readily separate it from other nearly allied species with which we are acquainted. EXPLANATION OF THE PLATES. PLATE LVI. Fig. 1 represents a portion of one of the pieces of Beanea favoides, exhibiting the dark sarcodous development of the skeleton on one part, and the denuded rotulate skeleton on the other portion, X 36 linear. 2. A small portion of the confluent rotulate skeleton of the specimen represented by fig. 1, showing the structure of the rotula** and the central canals of the fibre, X 80 linear. 3. Farrea inermis : a portion of the skeleton, exhibiting its general structure and the paucity of its armature, and a small piece of its membranous structure, with numerous gemmules embedded upon it at a, X36 linear. 4. The group of gemmules represented at a, fig. 3, X 80 linear. PLATE LVII. Fig. 1 represents a portion of the skeleton of Farrea per armata, with the manner of the disposition of its abundant armature, X 6 1 linear. 2. A small portion of the specimen represented by fig. 1, exhibiting the structure of the defensive organs, X 123 linear. 3 represents a portion of the skeleton of Farrea irregularis, exhibiting the intricate and irregular structure of its rete, X 31 linear. 4, a, b, c. Examples of the varieties in size and of the distortions of form of the interstitial sexradiate defensive organs, X 1 2 3 linear. |
