504 MR. A. DENDY ON THE WEST-INDIAN CHALININ^. [June 23, 



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. 



Tlie 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 desi- 

 rable 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 species \ 



Genus Pachychalina, Schmidt. 

 Pachychalina 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. 



' All the specimens were examined in the dry condition. 



