a CRISTA. 59 



TOOTHED. 



2. AcAMARCHis dentata. Two teeth on the exter- 

 nal side of the cells ; opening toothed. Plate 3. fig. 3. 

 a. B. 



Australasia. 



CRISIA. 



Plant- LIKE Polypidom. Dichotomous or branch- 

 ing ; cells slightly projecting, alternate, rarely oppo- 

 site, mth their opening on the same surface. 



All the Crisias have so much analogy of form 

 with each other, that these Polypidoms are easily to 

 be distinguished. 



Their substance is in general calcareous, with arti- 

 culations more or less horny. 



In the dried specimens, the colour slightly varies ; 

 it is usually a dirty white, but sometimes quite pure, 

 and at others tinged with yellow or violet. Their 

 height is generally from four to six centimetres ; in 

 some species it reaches to a decimetre, but I never 

 met with any exceeding that size. 



The Crisias, unlike the Cellarias, which shun the 

 Thalassiophytes, exclusively prefer these vegetables, 

 which they embellish with their white cretaceous 

 tufts : they are found, in the warm and temperate 

 regions, at all seasons of the year ; but in the colder 

 climates, they are rarely found during winter; and 

 their existence in all places appears to depend on 

 the marine plant to which they are fixed. 



