On the Lower Jaw in Rhizodopsis and Rhizodus. 299 



altogether the flesh-spicule. In them the horny element is 

 rather more developed than in Dr. Allen's sponge, and it yet 

 remains to be considered how far they are related to H. hiro- 

 tulataj they will therefore be more particularly referred to 

 when the rest of the collection comes to be described. 



EXPLANATION OF PLATE XIV. 



Mg. 1. Hi(/gimia coralloidcs, half actual size, after a photograph by Mr. 



John Chard, Liverpool Museum. 

 Fig. 2. Smooth beut acerate skeleton-spicule of same, scale 0*001 to 



0-0625 iuch. 

 Fig. 3. Smooth straight acerate subskeletou-spicule of same, scale O'OOl 



to 0'0625 inch. 

 Fig. 4. Spined bent acerate surface-spicule of same, scale 0"001 to 0-0625 



inch. 

 Fig. 5. Same spicule, scale 0-001 to 0-125 inch. 

 Fig. 6. Smooth couically spined stellate spicule of Donatia parasitica^ 



scale 0-0002 to 0-083 inch. 

 Fig. 7. Spino-capitately rayed spicule of same, scale 0-0002 to 0-083 inch. 

 Fig. 8. Subterminally inflated spiuulate spicule of same, scale 0-0004 to 



0-0416 inch. 

 Fig. 9, Entirely spined quadriradiate spicule of Hymeraphia unnamed, 



scale 0-0002 to 0-041(3 inch. 

 Fig. 10. Bent acuate spicule of same sponge, scale 0-0004 to 0-0416 inch. 

 Fig. 11 . Halichondria hirotulata, short branch, actual size, from a drawing 



hj my daughter, Eva Higgin. 

 Fig. 12. Subcylindrical skeleton-spicule of same, scale 0-0004 to 0-0626 



inch. 

 Fig. 13. Acuate subskeleton-spicule of same, scale 0-0004 to 0-0625 inch. 

 Fig. 14. 12-rayed birotulate flesh-spicule of same, five rays only at each 



end sho-wn, to avoid confusion of lines; scale 0-0005 to 1 inch. 

 Fig. 15. End ^-iew of one of the umbrella-shaped extremities of same 



spicule, scale one 1900th to 1 inch. 



XXV. — On the Structure of the Lower Jaio in Rhizodopsis 

 and Rhizodus*. By R. H. Traquaie, M.D., F.G.S., 

 F.R.S.E., Keeper of the Natural-History Collections in the 

 Museum of Science and Art, Edinburgh. 



Among the detached and broken-up remains of the Coal- 

 measure fish known as Rhizodopsis sauroides^ one of the most 

 frequently observed is a bone of a somewhat narrow and 

 elongated form, truncated and somewhat expanded at one 

 extremity, which may be assumed to be the anterior, and 

 pointed at the other or posterior. One margin, nearly straight, 



* Read before the lloyal Physical Society of Edinburgh, Feb. 2J, 1877. 



