4 Prof. H. A. Nicholson on the Structure and 



(h) In a second group of specimens tlie skeleton is more or 

 less extensively composed of phosphate of lime, and the 

 chambers of the fossil may be occupied, throughout or in part, 

 by phosphatic infilling. The greater proportion of the speci- 

 mens which I have examined are in this condition ; but the 

 extent to wliich the skeleton is phosphatic varies greatly. In 

 some examples the chambers of the fossil are filled throughout 

 with phosphatic material, and the whole skeleton seems to be 

 more or less largely made up of phosphate of lime. Even in 

 such specimens^ however, a certain amount of carbonate of 

 lime is present in the skeleton, since the application of a 

 drop of acid to a thin section of an apparently altogether 

 phosphatic specimen is followed by an evolution of carbon 

 dioxide. In most of the specimens belonging to this group, 

 however, it is only pa7't of the skeleton which is in the con- 

 dition of phosphate of lime, and the remainder is in the con- 

 dition of carbonate of lime. In such specimens it is invariably 

 the outer portion of the specimen, for a zone of greater or less 

 depth, which is phosphatic and has its chambers filled with 

 phosphatic material, while the i7iner or central part of the 

 specimen is composed of carbonate of lime and has its cham- 

 bers filled with calcite. 



(c) In a third group of specimens the skeleton is more or 

 less largely composed of phosphate of lime and the central 

 portion of the fossil^ or the whole of it, has its chambers empty 

 and non-infiltrated. 



[d) In a fourtli group of specimens the chambers of the 

 fossil are infiltrated with silica. I have never seen an 

 example in this condition of preservation ; but such a speci- 

 men is stated by Dr. W . B. Carpenter to exist in the Museum 

 of Practical Geology in Jermyn Street. The condition must, 

 however, be one of great rarity. 



The question now^ arises. What was the original composi- 

 tion of the skeleton of Farherial Dr. W. B. Carpenter 

 regarded the specimens of the third of the above-mentioned 

 groups as those least altered from their original constitution. 

 He therefore considered that the skeleton was composed of 

 a small proportion of sand-grains cemented together by a 

 mixture of phosphate and carbonate of lime. Mr. Carter, on 

 the other hand^ regarded the phosphatic condition of the 

 skeleton as of secondary origin and as being due to minerali- 

 zation subsequent to fossilization. He appears to think that 

 the skeleton may have been originally chitinous in nature, 

 and that the chitine may have been replaced during fossiliza- 

 tion by calcspar, which in turn might be more or less largely 

 replaced at a later stage by phosphate of lime. My own view 



