W. H. HARRIS OX MARINE MICROSCOPIC VEGETABLE ORGANISMS. 149 



some reason they are very rarely invaded by the j^lants,. although 

 they may be abundantly represented in other remains contained 

 in the same gathering. One valve only has been thought worth 

 preserving. 



But evidence of partiality may be adduced tending to show 

 that certain species select particular layers of shell for their 

 habitat ; fragments of shell may be met with which consist of 

 two or more distinct laminae of decidedly diJSerent structure and 

 colour: such fragments are selected by Achhja gracilis and A. 

 flexuosa. These plants spread superficially through the colour- 

 less, nacreous layer ; they give off short branches w^hich terminate 

 in variously-shaped bulbous cavities ; these plunge vertically into 

 the shell, but never go beyond the limits of this particular layer. 

 Other branches are produced which pass through the entire 

 thickness of the shell ; they are simple unbranched filaments 

 entirely unlike the plant from which they arise. Ranuncidus 

 aquatilis, with its two forms of aerial and submerged leaves, is 

 cei'tainly not more remarkable in this respect than is this tiny 

 organism. 



In such a piece of shell I once had a specimen of Varneia 

 villosa ; it was situated about equidistant from either surface ; by 

 applying pressure with a needle I was fortunate in causing the 

 two layers of shell to separate. I found the plant was wholly 

 confined to one of these, and although the newly-exposed surface 

 of the portion of shell containing the plant was eroded, no trace 

 of its having penetrated to the second lamina could be detected. 



Comjjosition of ^Shells. 



Any one who devotes any study to dredgings cannot fail to 

 be impressed with the different appearances the fragments of 

 moUuscan shells present ; from the highly prismatic shell of the 

 Pinna to the porcellaneous foraminifer with its homogeneous 

 composition, every gradation of structure may be obtained ; thus 

 they may be fibrous, laminated, horny, or glossy and translucent — 

 in lustre they may be dull or nacreous. 



The animal matter present also varies chemically, as well as in 

 the method of its employment in the building of the shell. 



In porcellaneous shells the animal matter consists of a small 

 quantity of soluble gelatine ; in nacreous shells it is albuminous ; 

 if the latter are immersed in very dilute hydiochloric acid they 



JouRN. Q. M. C, Series II.— No. 44. 11 



