430 Prof. J. Young and Mr. J. Young on 



with siliceous matter which had or acquired the granular con- 

 dition, and which has since been removed, leaving a hollow 

 core to the incrustation [cf. PL XIV. figs. 5, 6). 



5. Unrecognized Forms. 



We have given a large number of drawings with the view 

 of calling attention to certain forms whose identity is obscure 

 or unknown to us. 



A. Figs. 38-40 are of objects which we at first thought to 

 belong to Haplif^tion\ but we are now doubtfid of the reference. 

 They are silicified plates enclosing irregular cavities. They 

 may be fragments of the skeleton of a hexactinellid sponge 

 whose nail-like spicules have become deformed by siliceous 

 deposit ; but the possibility of their radiolarian character has 

 been suggested. 



B. The series figs. 42-51 represents objects which have 

 some resemblance to part of the foregoing; but their variety is 

 considerable, and we have not space to illustrate all the types. 



C. The stellate spicules (figs. 13, 18, 19, 24, 26, 27, 29) have 

 probably dropped out from other sponges. These spicules are 

 of three kinds : — 1, smooth (figs. 13, 18, 19, 26) ; 2, pustu- 

 late (figs. 27, 29) ; 3, cushioned (fig. 24). It is impossible to 

 refer them to any of the species we have described ; they may, 

 indeed, be parts of forms not yet recognized. 



Geological Position of the Sponges. 



The Carboniferous Limestone in "West Scotland is divided 

 into an upper and lower series by a mass of coals, ironstones, 

 and shales, which are in part replaced in North Ayrshire by 

 volcanic rocks. The Lower Limestones are again divided 

 into an upper and lower group by from 10 to 40 feet of shales. 

 The beds at Cunningham Baidland in which the sponges 

 were found belong to the upper member of the Lower Lime- 

 stones, in which group also volcanic rocks occur. The 

 limestones show, when undenuded, a thickness of 40 feet (R. 

 Craig " On the Carboniferous Basin of Dairy," Trans. Geol. 

 Soc. Glasgow, iii. p. 275) ; but at Cunningham Baidland the 

 quarry-section shows only between 20 and 30 feet. The rock 

 is very much jointed, traversed by vertical fissures ; and 

 slicken-sided faces abound. At the north end of the quarry 

 a narrow dyke of dolerite runs nearly east and west. These 

 limestones are among the most fossiliferous in West Scotland j 

 but the organisms are not equally distributed. Certain layers 

 abound in the remains of Corals of difierent genera and species; 

 in others Brachiopods, ProductcBj &c. are the chief feature. 



