G. ISIS. 235 



two fold circumstance of the thickness of the rind 

 and its adherence to the axe. 



These Polypidoms vary but little in their form, 

 being always cylindrical, with branches dispersed. 



Neither in colour do they differ materially ; they are 

 whitish when the Polypidom is clothed w ith the rind ; 

 but in the axe two very distinct shades are ob- 

 servable ; the calcareous articulations having a white- 

 ness rembling rock-salt or marble, from its semi- 

 transparent purity ; whilst in the horny divisions the 

 colour is sometimes brown of various hues, some- 

 times yellowish, and sometimes almost black. 



Their height varies from one to five decimetres. 



These Polypidoms, inhabiting all seas, are found 

 on the coasts of Iceland, as well as those of the 

 equatorial regions ; there are but few species known, 

 but the greater part are said to originate in the 

 Indian Ocean. 



HORSE-TAIL. 



1. Isis hippuris. Stony articulations, irregular, 

 striated longitudinally, those at the extremities com- 

 pressed, sometimes almost flat. 



In all seas. 



ELONGATED, 



2. Isis elongata. Branches very few, and those 

 frequently anastomosed; stony articulations, long, 

 cylindrical, deeply striated, and larger than the horny 

 articulations ; height six decimetres. 



Indian Ocean. 



Nota. In this species, and the preceding, the stony 



