On the Lower Jaw in Rliizodopsis and Rliizodus. 299 



jilto<;etlier the flc.sli-s])icule. In tlieni the horny element is 

 rather more deveh^ned than in Dr. Allen'.s sponge, and it yet 

 remains to be considered how far tliey are rehited to //. biro- 

 tiilata ; they will therefore be more particularly referred to 

 when the rest of the collection comes to be described. 



EXPLANATION OF PLATE XIV. 



Fiy. 1 . Ilujgitma coralloidcs, half actual size, after a photograph by Mr. 



John Cluird, laverpool Museum. 

 Fii/. 2. iSmooth bent acorato skeleton-spicule of same, scale 0"(X)1 to 



O-OC)-'.") incli. 

 Fiy. .'{. Smooth straight acerate subskeletou-spicule of same, scale 0-001 



to OU(52o inch. 

 Fiff. 4. Spint'd bent acerate surface-spicule of same, scale OOOl to 0-0625 



inch. 

 Fiff. 5. Same spiculf, scale 0-(X)l to 0-125 inch. 

 Fiff. (j. Smooth conically spined stellate spicule of Donatia parasitica 



scale 0002 to 0-08.'} inch. 

 Fig. 7. Spino-capitately rayed spicule of same, scale 00002 to O-OS."] inch. 

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



0041G inch. 

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



scale 00002 to 0041G inch. 

 Fig. 10. Bent aciiate spicule of same sponge, scAle 0-0004 to 0-041G inch. 

 Fig. 11. JIalichonrlria birutuiata, short branch, actual size, from a drawing 



by my daughter, Eva Iliggin. 

 Fig. 12. .Subcylindrical skeleton-spicule of same, scale 0-0004 to 00026 



inch. 

 Fig. 13. Acuate subskeleton-spicule of same, scale 0-0004 to 0-0(325 inch. 

 Fig. 14. 12-raved birotulate flesh-spicule of same, five ravs only at each 



end sliown, to avoid confusion of lines; scale 00005 to 1 inch. 

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



spicule, scale one 1900th to 1 inch. 



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

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

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

 Museum of Science and Art, Edinburgh. 



Amo.nt} the detached and broken-up remains of the Coal- 

 measure fish known as Ilhizodopsit sauroides, one of the most 

 frequently ob.scrved 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, jlnd 

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



* Read boloro tlie Unyal Physical Society of Edinburgh, Feb. 21, 1877. 



