292 The American Naturalist. [April, 
flinty sponges is removed, and its place taken by carbonate of 
lime, or by oxide and sulphide of iron. The secondary char- 
acter of this lime seems also proven by the fact that it is always 
crystalline, and its crystals are placed confusedly in all direc- 
tions, and not in one, or, as it is technically expressed, are 
unoriented. 
Prof. Nicholson, in his study of certain obscure organisms 
known under the general designation of Stromatoporoids, has 
indicated three different conditions or phases of their preserva- 
tion. These organisms are in the main calcareous encrusting 
or turbinate masses, built up by a succession of poriferons 
sheets or lamine, between which irregular spaces extend, fur- 
rowed by inosculating canals. In the first state of preservation, 
instanced by this author, the actual calcareous skeleton is pre- 
served, and all cavities are infiltrated with transparent calcite, 
the skeleton then appearing as a brown granular or cloudy 
non-crystalline body. In the second method of fossilization 
more or less silicification has taken place, the cavities becom- 
ing solidly silicified and the skeleton remaining calcareous, 
or the skeleton irregularly presenting a complete siliceous 
frame-work. In the third method the specimens are preserved 
in limestones or in argillaceous deposits. The skeleton became 
infiltrated with fine mud or argillaceous sediment, and was 
dissolved out, being the less stable form of carbonate of lime— 
aragonite—and was replaced by calcite. Thus, the skeleton 
appears as clear as calcite, while the chambers, pores and canal- 
system of the fossil are represented by comparatively opaque 
calcareous mud or fine argillaceous fillings. In the skeletons 
and hard parts of living invertebrates the following distinc- 
tions of mineral composition have been determined : 
The calcareous foraminifera are composed of calcite, with 
some aragonite. 
The true corals are composed almost entirely of aragonite. 
The alcyonarians are for the most part composed of calcite, 
with small amounts of aragonite and phosphate of lime. 
The echinoidea are essentially formed of calcite. 
The annelids are enclosed frequently in tests, tubes, or shells 
made of calcite. 
