14 Mr. H. J. Carter on three Pachytragous Sponges. 



EXPLANATION OF PLATE IV. 



N.B. Figures 3, 9-11, and 17-19 are all drawn to the same scale, viz. 

 l-24th to l-1800tb of an inch; and figures 4, 5, 12, 13, 21, and 22, also 

 to the same scale, viz. l-6th to l-6000th of an inch, to show their rela- 

 tive sizes respectively. 



Fig. I. Dercitus niyer, n. var. : section of a portion, natural size, show- 

 ing sui-face (a), black cortical layer (h), and body-substance (c) 

 presenting truncated ends of excretory canals. 



Fig. 2. The same, diagram section, much magnified, showing dermal 

 layer charged with minute spinous spicules (a), black cortical 

 layer composed of cells ( b ), layer of trifid spicules (c) , body- 

 substance {d) less densely charged with the trilid spicules, 

 together with minute tricurvate or bow-like spicides, and black 

 cells. 



Fig. 3. The same, trifid spicule with minute dermal spinous (a) and tri- 

 curvate (6) ones, together with black cells (c), all relatively 

 magnified. 



Fig. 4. The same, dermal spicule, gi'eatly magnified. 



Fig. 5. The same, tricurvate spicule, greatly magnified, on the same scale. 



Fig. 6. The same, black cell, still more magnified, showing large sub- 

 circular nuclear (?) body (a), small nucleolar (?) body (b), mi- 

 nute globular cellules (o), charged respectively with one or more 

 black or dark-brown gTanules, which collectively give the dark 

 colour to the cell, and the latter collectively the intense black 

 colour to the cortical layer ; d, separate cellule. 



Fig. 7. Stclletta aspera, u. sp. : section of a portion, natural size, show- 

 ing surface (a), cortical layer (b), and body-substance (c), pre- 

 senting truncated ends of excretoiy canals. 



Fig. 8. The same, diagram section, much magnified, showing dermal 

 layer charged with minute sub-bistellate spicules (a), cortical 

 layer of cells (b), body-substance (c) charged with large acerate, 

 trifid, fm-cate, and minute stellate spicules, of which the fomier 

 are most aggregated towards the surface, where they are inclined 

 so much as to be almost horizontal. 



Fig. 9. The same, large acerate fusiform spicule ; viewed from above, 

 straight ; but slightly curved when viewed laterally : a, real 

 length. 



N.B. For convenience, this spicule has been drawn from that 

 point of view in which it appears straight, as the curved form, 

 when slight, is so diflicult for an engraver to etch truthfully ; 

 also, to make this easier, its sides have been drawn with a rule, 

 diminishing equally from the centre to the extremities, which 

 is not the case with this nor, indeed, with most spicules, which 

 diminish more abruptly towards the ends than in any other part. 

 These observations will apply also to the representation of the 

 large acerate spicule in the next species. 



Fig. 10. The same, trifurcate spicule, lateral view : a, real length. 



Fig. 11. The same, trifurcate head, of larger size, where the rays are more 

 deeply furcated, end view : a, end of shaft ; b, central canal, 

 bifurcating like the arms. 



Fig. 12. The same, dermal spicule, much more magnified. 



Fig. 13. The same, stellate spicule of the body, magnified on the same 

 scale. 



Fig. 14. The same, three cells of the cortical layer, less magnified than 

 fig. 6, the same as those of Dercitus tiiger, only colourless, show- 



