224 DR. W. B. CARPENTEft. 



unhesitatingly referred to that genus by Prof. Sars, who 

 , has kindly transmitted to me examples of them, f The variety 

 in the composition of the test would thus seem referable to 

 the nature of the material furnished by the bottom on which 

 these Rhizopods live, and partly to the condition of that 

 bottom as regards' stillness. We have seen that there are 

 some arenaceous Foraminifera whose very symmetrical tests 

 seem only capable of being built up where materials can be 

 found of a precisely suitable kind, whilst there are others 

 which appear able to use for their ruder constructions any 

 material which the bottom may happen to supply; and 

 Astrorhiza obviously belongs to the latter category. But 

 the most curious difference betAveen the original specimens 

 and those more recently collected by M. Sars and ourselves 

 lies in the manner in which these materials are united. For 

 whilst in the latter the sand -grains are so slightly connected 

 together by any intermediate material that the test is very 

 easily disintegrated, the Avail in the former has at the same 

 time a considerable amount both of tenacity and flexibility — 

 in this last particular differing from all other arenaceous 

 tests Avitli which I am acquainted. Noav the extreme brit- 

 tleness of the Astrorhiza brought from great depths, Avhere 

 a profound stillness prevails, would be speedily destructive 

 to them if they lived amidst the agitated waters of the 

 surface ; and it Avould seem as if the animal had the poAver, 

 by exuding an organic cement, of so uniting the materials of 

 its test as to give it just that poAver of resistance Avhich is 

 most suitable to its habitat. 



