CIV INTRODUCTION. 



calcification is carried to a great extent, and proceeds 

 with much rapidity; the cell is buried beneath a thick 

 mass of stony matter, which in a short time obliterates 

 its characteristic features, and completely changes its 

 aspect. In such cases the original orifice of the cell is 

 found to be placed at the bottom of a deep shaft, piercing 

 the vitreous crust which has been piled on the surface- 

 It is difficult to understand the mode in which this rapid 

 accumulation of stony matter takes place, and the con- 

 ditions which favour it. When the calcareous elements 

 are discharged by immersion in acetic acid, the wall is 

 represented by a thick stratum of animal matter. Physio- 

 logically the subject is of high interest, whilst to the 

 systematist the effect of the calcification in modifying 

 the characters of the species is a point of the first impor- 

 tance. 



I shall add here an account of the development of the 

 cell in Mucronella coccinea, one of the incrusting forms, 

 so far as the external portions of the structure are con- 

 cerned. 



First stage. Membraniporidan condition : a rhom- 

 boidal cell, with its aperture covered over by a flat mem- 

 branous roofing. Second stage. A delicate, transparent, 

 membrano-calcareous arched covering, in course of de- 

 velopment over the aperture from the bottom of the cell 

 upwards, marked with transverse striae. Before this has 

 spread over half the front surface, four spines are developed 

 at its upper extremity (Woodcut, fig. xliv.). Third stage. 

 Front wall completed, arching over the whole aperture, 

 with the exception of a subcircular space at the top, which 

 is left open ; the membrane covering this free space takes 

 on a decidedly corneous character, and constitutes the 



