174 Mr. H. J. Carter on Gjpsina melobesioides 



my Polytrema planum, which in name now becomes quite as 

 inappropriate as " Tinoporus^'' must also be suppressed for 

 Oypsina melobesioides, since it is but a small specimen of the 

 latter, and no Polytrema at all, as the sequel will show. 



There is a very common sponge to be obtained from the 

 shores of the Mauritius, which evidently belongs to my group 

 " Otahitica^'^ but differs from the typical structure of the 

 Psammonemata in being composed of simple fibre coated, 

 instead of " cored," with foreign bodies, wherefore a sub- 

 group will have to be formed for this and the like sponges, 

 which might be termed " Sarcopsammosay Be this as it 

 may, this sponge is found growing on old coral detritus, 

 from a point, which, increasing in size, soon divides into several 

 roundish stems, each of which becomes compressed, and, again 

 dividing, anastomoses with the neighbouring branches, finally 

 terminating in thin, spatuliform, leafy expansions, each of 

 which is surrounded by a somewhat inflated margin. The 

 surface of the branches thus flattened and lacinulated is tra- 

 versed on each side by a superficial vein-work or reticulation, 

 which is the excretory canal-system ; and the whole thus 

 presents the character of the well-known Spongia othahetica of 

 Solander and Ellis, whose colour, when dry and shore-washed, 

 is whitish yellow, like that of cream. For this sponge and 

 its like I would propose the generic name of " Mauricea^'' 

 and for the species M. lacimdosa. 



Like many other sponges it is frequently more or less in- 

 fested by small parasitic Balani, which, ensconced in little 

 round processes of sponge substance open at the summit, are 

 supplied from the sponge itself and more or less scattered over 

 its branches ; while the stems are as often beset with Polytrema, 

 Planorbidina, and worm-tubes. 



It is these stems which are covered in the present instance 

 with Gypsina onelobesioides, continuously, to the extent of three 

 inches from the base. There are 8-10 stems, averaging in 

 their round part a quarter of an inch in diameter, and in their 

 flattened or compressed form above about half an inch in their 

 longest diameter. The thickest part of the fbcrustation, 

 which has very much the appearance of the white saccharine 

 layer spread over bridecakes (hence the name Gypsina), but 

 with a faint bluish tint like that of snow, owing to the crys- 

 talline transparency of its cell-structure, is about one forty- 

 eighth of an inch ; and the number of layers of which this is 

 composed varies from ten to twenty. I need not go into the 

 minute structure of Gypsina melobesioides, which is precisely 

 the same as that of Polytrema planum and the spheroidal 

 variety of Tinojjorus vesicularis, to which I have alluded — 



