HALIOTIS. 279 



examined with a sufficiently high magnifying power, 

 indicate a minute cellular structure, such as I have not 

 observed in the nacre of bivalves. The cells are of a 

 long oval form, and their short diameter is not above 

 ?r<K>()tn of an inch. Their boundaries in many parts 

 are very indistinct or even disappear altogether ; so that 

 every gradation can be traced, from the obviously cel- 

 lular arrangement to the homogeneous appearance pre- 

 sented by the nacre of bivalve shells. Hence I should 

 be disposed to draw the same inference, with respect to 

 the nacreous structure, as in regard to other forms of 

 apparently homogeneous shell-substance namely, that 

 like them, it was originally formed upon a cellular plan, 

 but that the cells subsequently coalesced, their bound- 

 aries disappearing." Woodward gives seventy-five as the 

 number of recent species, and four for the fossil 

 (miocene) species. The distribution of this genus com- 

 prises every part of the ocean, from Great Britain 

 southwards. 



It is the genus Auris of Klein, 



* as 8 HALIOTIS TUBERCULA'TA *, Linne. 



H. tuberculata, Linn. S. N. p. 1256 ; F. & H. ii. p. 485, pi. Ixiy. and 

 (animal) pi. C C. f. 3. 



BODY mottled with brown, green, and white, blending agree- 

 ably together : foot ornamented with two rows of most deli- 

 cate thorn-like processes, which alternate with green filaments ; 

 the interspaces between these rows are covered with greenish 

 tubercles ; sole of a salmon-colour. 



SHELL much depressed, solid, opaque, and of a dull aspect : 

 sculpture, numerous narrow longitudinal grooves or striae, 

 which are more or less close-set and occasionally undulating ; 

 the surface exhibits also the edges of extremely minute trans- 

 verse plates, that compose the structure of the shell ; marks 

 of growth sometimes fold-like : colour reddish-brown, mottled 



* Tuberculated. 



