SPONGES. 99 



Grade : LITHISTIDA. 



Tetraxonida in which the megascleres form desraas, typically united with each other 

 to form a continuous skeleton. 



'Hie relationships and classification of the Lithistida still require a great deal of 

 investigation before we can consider our knowledge of the group as by any means 

 satisfactory. They are. of course, usually considered as a sub-division of the 

 Tetractinellida, but inasmuch as many of them have only monaxonid spicules this 

 method of disposing of the group seems somewhat unjustifiable. There appear to 

 lie two logical alternatives. We may either regard the Lithistida as being of 

 polyphyletic origin and partition them to the best of our ability amongst other 

 groups, or we may regard them as representing another grade of evolution in a 

 special direction and retain the group provisionally as a matter of convenience, 

 without committing ourselves to an opinion as to whether it is a natural group or not. 

 In the present state of our knowledge the latter appears t<> me the wisest course 

 to adopt. 



As regards the internal classification of the group, I have no doubt that important 

 modifications of the existing arrangement of genera, which we owe to Sollas, must 

 shortly be made. My own experience of the Lithistida is, however, so limited that 

 I prefer not to experiment in this direction, but will content myself with a few 

 critical remarks suggested by the investigation of certain species in Professor 

 Herdman's collection. 



Discodermia, Bocage. 



Lithistida with tetracrepid desmas and ectosomal discotriienes, and with microscleres 

 in the form of microxea or microstrongyla. 



Discodermia emarginata, n. sp. Plate IV., fig. 4. 



The single specimen is of massive form with spreading base attached to a mass of 

 calcareous worm-tubes. The upper surface is strongly convex, rising into two low, 

 mammiform projections, from the broad apices of which (where the small vents are 

 situated) radiate shallow grooves, the outward indications of convergent exhalant 

 canals. Colour throughout (in spirit) pale yellow; texture compact and stony. 

 Diameter of massive central portion about 12 millims., with the basal portion 

 sj Heading a good deal further as a thin crust. 



Skeleton composed of a close reticulation of firmly united tetracrepid desmas witli 

 a single layer of discotriames on the surface. Below the surface are a few slender 

 oxea, which become more numerous and are sometimes collected into loose fibres in the 

 neighbourhood of the vent. 



Spicules: (1.) Stout tetracrepid desmas (Plate IV., fig. 4, a-e), more or less 

 tuberculated. especially at the ends of the branches, which proliferate into numerous 

 rounded tubercles, by the interlocking of which the union of adjacent desmas is. 



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