Mr. II.. I. CiirttT on two new Calcis|x»iigije, /) 



" Mr. MKlolla has funji.shed me with a vairirtv tVoiii the 

 Irish coa-xt tliat nierit.>< to W di.stin^ui.-^lucl. 'V\\v .x|)(iii',a* rises 

 up in coinpri's.sccl simiou.s h'at-liki- loins, wliich arc united 

 together so as to t'orni a Inlnihited crust nearly an inch in 

 thickness, with a circular osculuni on every jjrojccting angle 

 (pi. 20. Hg. iSy Were we to inuigine that a' cluster of </r««^a 

 comjtrfs.su had grown so close as to j)ress against each other, 

 and the various specimens to have coalesced into one mass, 

 •vse would have a coiTect idea of this variety. That it is, 

 however, no variety of G. co/njtressa, is proved by the diiier- 

 ence of its texture as well as by the form of the spicula." 

 (Brit. Snong. 1842, p. 1S3.) 



I need hardly add, after this quotation, that Dr. Johnston 

 was accjuainted with the species Avhich 1 have now the plea- 

 sure to dedicate to his respected memory ; nor, on the other 

 hand, need 1 allude further to Dr. Bowerbank's description of 

 Leuconia nirea (Brit. Spong. 18(36, vol. ii. p. 36) than to state 

 that, as he has changed Johnston's name of Grantia nivta to 

 Leuconia ni'ceo, so he has lost sight of or ignored this classical 

 writer's description of the true Grantia nivea, and replaced it 

 by an imperfect one of his " variety." 



That, however, Johnston's " variety" is entitled to a distinct 

 appellation, the above description will show. 



As the great quadriradiate spicule of the surface oi Leuconia 

 Johnston ii is but a larger fonn of that which is common to the 

 cloacal surface alone of most of the calcareous sponges, I have 

 given an illustration of that which is found in Grantia ciliata 

 as a type specimen (PI. 11. fig. 32). It will be observed that 

 one ray is straight {h), while two others are more or less 

 curved and opposite to each other (a a) ; this is the common 

 form of the /r/radiate spicule ; and it is in the straight ray 

 alone that a trace of the central or axial canal common to the 

 siliceous spicule is seen (c), which trace, however, is here the 

 central canal filled up with a cylinder of the same material as 

 the spicule, so that, in fact, there is no canal at all. The 

 fourth ray {d) projects at about right angles to the other three, 

 and sometimes is a little excentric — that is, arises from the 

 straight ray at a little distance from its union wdtli the two 

 curved ones. This ray is also cm'ved forwards (that is, towards 

 thevent), and in this way projects into and forms the armature 

 of the cloaca : it would have been opposite, probably, if the 

 current had been so, and hence is one of the structural evi- 

 dences of an aboriginal excretory stream. 



What is remarkable, however, in Leuconia Johnstonii is, 

 that this spicule is so large that its fom-th ray not only pro- 

 jects in a formidable manner into the cloaca (PI. II. fig. 40, c c), 



