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Show 504 MR. A. DENDY ON THE WEST-INDIAN CHALININ^S. [Juiie23, But this law was illustrated in a still more striking manner by two species of the genus Spinosella, viz. Spinosella plicifera, D. & M., and Spinosella maxima, n. sp., which sometimes still contained traces of spicules imbedded in the horny fibre, and apparently on the verge of disappearance, while at other times they contained no spicules whatever, the skeleton-fibre being entirely horny ; and yet specimens with spicules and specimens without were specifically undistinguish-able. It appeared that spicules might persist as vestigial structures long after they had ceased to be of any functional importance, and that they disappeared first from the secondary fibres of the skeleton. The bearing of these facts upon the systematic position of the so-called " Keratosa " was pointed out, and was, indeed, sufficiently obvious. The immediate cause of the disappearance of the spicules appeared to be the development of the spongin to such an extent as to form by itself a sufficiently strong skeleton. In such a skeleton spicules would probably be not only useless, but actually harmful, in that they would tend to make the fibre rigid and brittle when it is desirable that it should be elastic and flexible, in order to facilitate the free contraction and expansion of the various parts of the canal-system, and in order better to withstand the action of the waves and currents in the shallow water in which horny-fibred Sponges occur. Spongin appeared to be developed to a large extent only in warm climates and in tolerably shallow water ; and under such conditions Sponges with a strongly developed horny skeleton are abundant. In the second part of the paper eight species were fully described, five of them being new. The following is an enumeration of the genera and species, together with brief diagnoses of the new species1. Genus PACHYCHALINA, Schmidt. P A C H Y C H A L I N A VARIABILIS, sp. n. External form extremely variable ; usually the same specimen is both lobose and digitate. Size of largest specimen 38 cm. high by 37 cm. broad. Colour (dry) light yellowish grey. Texture hard, fibrous, somewhat elastic. Surface smooth. Oscula large and round, about 5 millim. in diameter ; for the most part scattered irregularly over one surface of the specimen. Main skeleton a very irregular reticulation of strong spiculo-fibre, containing both a large amount of spongin and a great number of spicules. Primary and secondary lines much confused, longitudinal fibres strongly developed. The dermal skeleton also forms an irregular network. Spicules long and very slender, normally oxeote or strongylote ; size 0-126 by 0*003 millim. Occurring in the fibre and scattered outside it. Nassau, Bahamas. 1 All the specimens were examined in the dry condition. |
