Carboniferous Sponges from Ayrshire. 429 



pod shells in tlie same deposit. The absence of a central 

 cavity may be due to pressure by which the canals in fig. 34 

 are reduced in number, and certainly such pressure would 

 affect a horny more readily than a siliceous sponge. The 

 original of fig. 31 is of a greyish-white colour,' and under the 

 microscope has the look of grey pumice, so dry and harsh does 

 it seem. We have been unable to separate fibres for exa- 

 mination by transmitted light, and doubt if they are sufficiently 

 translucent to show any thing. 



We have dedicated the species to Mr. J. Armstrong, a 

 Glasgow palaeontologist, to whom we owe this and many other 

 interesting forms.. 



3. A Species of the Group Gummina. 



The spicules (figs. 18,. 28, 42, 43) so closely resemble the 

 biternate spicules figured by Schmidt and others from Corti- 

 cium candelabrum^ that, though no birotulate spicules have 

 been found, we think ourselves justified in referring these 

 fossils to an allied genus. They are of gigantic size as com- 

 pared with living forms ; but they are so important a feature of 

 the rotten limestone that we would name them provisionally 

 Chlamys magna^ so as to help future collectors and, it may be, 

 hasten their identification with the parent organism, though the 

 composition of the Gummina renders tliis last improbable. 

 The drawings for this paper were completed in February last, 

 before we had recognized their probable nature. We have 

 now large series of specimens, which we shall illustrate later. 



Locality. Cunningham Baidland. 



4. Incrusting Sponge"^ 



Plate XV. fig. 41 represents a curiously branched fragment 

 which one of us (Mr. Young) thinks may be an incrusting 

 sponge encasing an organism whose decay has left a hollow 

 cast. These fragments are numerous, and present considerable 

 diversity of form ; but their structure is the same, granular and 

 tinged yellow with iron. They have evidently undergone 

 considerable metamorphism. 



Mr. Carter has described (' Ann. & Mag. Nat. Hist.' 1873, 

 xii. p. 458) a condition of the vitreohexactinellid sponges in 

 which the vitreous fibre has become hollow by absorption of 

 the hexactinellid spicules, and even of the vitreous fibre itself 

 in part. There is no proof of any Carboniferous sponge 

 belonging to the Aph-ocaUistes group ; but it is possible that 

 our fig. 41 may represent a fragment of Haplistion incrusted 



