CONDITIONS OF FOSSILIZATION 287 



of calcite may remain. In some strata the aragonite skeletons 

 have entirely disappeared. This is most likely to occur in pervious 

 beds. The presence of calcite forms does not necessarily imply 

 that they were not associated with aragonite forms. The conditions 

 of preservation , also vary. In the Mesozoic clays we find cephalo- 

 pods as originals, while in the Palaeozoic clays they are calcite casts. 

 Mytilus edulis secretes aragonite as its inner layer and calcite as 

 its outer layer. Fossils occur in which the inner layer is gone. Cal- 

 cite replaces aragonite at times; but in such cases the internal 

 organic structure is gone. As yet no example of aragonite replacing 

 calcite has been reported. 



Under Table I is given the horizon, locality, and lithological 

 character of the formations studied, and also the class, conditions 

 of preservation, and remarks in connection with certain forms found 

 in these formations. 



GENERAL MINERAL CHARACTER OF LIVING INVERTEBRATES 



Foraminifera. — The vitreous and porcellanous forms are calcite. 

 The arenaceous forms are siliceous throughout, or have a sandy- 

 siliceous layer incrusting an interior calcareous layer. The Gro- 

 midae are chitinous. 



Radiolaria. — Some are composed of acanthine and some of silica. 



Spongiae. — The Myxospongiae are composed entirely of soft 

 tissues. The Cerato spongiae are made up of spongin fibers. The 

 Silici spongiae are made up of siliceous elements or contain siliceous 

 spicules. The Calcispongiae contain calcareous spicules. 



Anthozoa. — The Madreporaria are aragonite, and the Alcyo- 

 naria are calcite. 



Hydrozoa.—The H ydrocorallinae are calcite ( ?) and the Tubu- 

 lariae calcite ( ?) and chitin. The Graptolitoidea are chitin. 



Echinodermata. — Calcite. 



Vermes. — Calcite ( ?). 



Bryozoa. — Calcite and aragonite ( ?). 



Brachiopoda. — Calcite. 



Lamellibranchiata. — Some are calcite, some aragonite, and some 

 both calcite and aragonite in layers. 



Scaphopoda. — Aragonite (?). 



