Carboniferous of the S.W. of Scotland. 135 
the rhombohedral excavations, which may appear singly in 
some, increased to a plurality in others, which not only has 
caused them to lose their original outline, but to become 
fretted into shapes which are chiefly characterized by the 
angular cavities caused by the encroachment of the calespar 
upon the siliceous material (fig. 14, ¢), so that a little more 
and the whole of the siliceous spicule would have given place 
to calcareous material. The calespar has become redissolved ; 
and the rhombohedral cavities which it occupied are thus left to 
prove the interesting fact first pointed out by Mr. W. J. Sollas, 
viz. that calcareous material, 7. e. phosphate of lime, might 
replace siliceous material in the ‘ vitreo-hexactinellid sponge 
Eubrochus clausus during fossilization’’ (‘Geol. Mag.,’ Sept. 
1876). This, which is one of the most important discoveries 
in modern paleontology, on account of the few organisms 
which possess siliceous skeletons, and the consequent rarity 
of the occurrence, while the reverse is so commonly the case 
with calcareous organisms that are replaced by silex, was 
subsequently put forth by Mr. Sollas in a more extended form 
in his paper on “ Pharetrospongia Strahani,’ read at the 
Geological Society on the 20th Dec. 1876, and published 
in May 1877 (‘ Quart. Journ. Geol. Soc.’ p. 242), which is 
supplemented by a “ Note,” dated “26th April,” in which 
(p. 254) Mr. Sollas, on account of the objections made to 
his view in the discussion of -his paper, states :—I need 
here only remark that while Siphonia exhibits the structure 
of a Lithistid (siliceous) sponge, Stawronema of a Hexacti- 
nellid (siliceous) sponge, and Pharetrospongia of a 'Thalyosian 
(siliceous) sponge, yet the fossil skeletons of all three fre- 
quently occur now in a calcareous state.” 
About the same time (remarkable facts are frequently noticed 
simultaneously by different observers independent of, and at a 
distance from, each other) Prof. K. A. Zittel of Munich must 
have come to a similar conclusion, as we learn from the 
“Note” to his paper on the Hexactinellida, entitled “Studien 
iiber fossile Spongien,” dated 15th Feb. 1877, wherein it is 
stated that at the general meeting of the German Geological 
Society, held at Jena in August 1876, he discussed the 
conversion of the originally siliceous skeleton [of the Hex- 
actinellida] into calespar, at which time, in the course of 
conversation, many objections were made to this chemical 
substitution (transl. ‘Ann.’ 1877, vol. xx. p. 516). The 
report of this meeting was subsequently published in the 
Zeitschr. d. deutschen geolog. Ges. xxvili. p. 6315; after 
which the paper above mentioned was read on the 13th Jan. 
1877, in the Mathem.-Physical Class, and finally published in 
