GENERA OF FAVOSITID^. 169 



periodical ring - shaped thickenings, which are placed at cor- 

 responding levels in contiguous tubes, in such a manner as 

 to leave vacant spaces between the intervening unthickened 

 portions. Visceral chamber in the outer portion of the tubes 

 alternately contracted and dilated in correspondence with the 

 periodical thickening of the walls of the corallites just spoken 

 of; but open and subpolygonal in the axial portion of the cor- 

 allum. Septa obsolete. Tabulae remote, usually placed at cor- 

 responding levels in contiguous tubes. Mural pores of small 

 size, not numerous, and irregularly distributed. 



History. — The genus Stenopora, Lonsdale, was first described 

 by its author in a note in Darwin's ' Volcanic Islands,' but was 

 more fully defined in Strzelecki's * Physical History of New 

 South Wales ' as follows : " A ramose spherical or amorphous 

 tubular polypidom ; tubes polygonal or cylindrical, radiated 

 from, a centre or an imaginary axis, contracted at irregular 

 distances, but in planes parallel to the surface of the speci- 

 men ; tubular mouths closed at final (?) period of growth ; 

 ridges bounding the mouths granulated or tuberculated ; addi- 

 tional tubes interpolated." 



It is quite clear, as properly pointed out by Dana (U.S. 

 Expl. Exped. Zoophytes, p. 537, 1848), that the above generic 

 diagnosis is insufficiently characterised ;^ and to this must be 

 ascribed the great confusion in which the genus has subse- 

 quently become involved. Professor Dana, in the work just 

 quoted, defines the genus as follows : — 



" Internal structure of corallum fine prismatic; cells of sur- 

 face minute, subangular, contiguous ; zoophytes glomerate or 

 ramose ; surface often small-verrucose." 



This definition, also, really adds nothing to our knowledge of 

 the actual structure of the genus ; and it is therefore no matter 



1 In the original description of Stenopora in Darwin's 'Volcanic Islands,' Mr 

 Lonsdale does give a character of generic value — viz., the gradual closure of the 

 mouths of the tubes by the deposition of calcareous matter on the interior of the 

 wall, giving rise to what he regarded as periodical "constrictions" of the tubes; 

 though these are really periodical " thickenings " of the wall, it being only the 

 visceral chamber that is " constricted." 



