AKD SPECIES OF HTDHOIDA. 267 



TRUIABIID^. 



Thuiaeia. 



The characters which have been hitherto employed in the defini- 

 tion of the genus Thuiaria are altogether inadequate. The species 

 have all a very distinctive aspect, and the whole facies of the forms 

 which are referable to this genus is so characteristic as at once to 

 strike the observer ; and yet in the attempts hitherto made to 

 define the genus not a single character has been introduced which 

 wiU not just as well apply to some of the typical Sertulariidans. 



The peculiarity which has been regarded as of sufficient import- 

 ance to constitute the essential character of the genus Tliuiaria 

 is the more or less completely adnate condition of the hydrothecse 

 to the hydrocaulus, which gives to these receptacles the appearance 

 of being immersed in the substance of the stem and branches. 

 The degree, however, in which this condition presents itself varies 

 greatly in the different species ; and the character is just as de- 

 cided in many species rightly referred to Sertularia or Sertularella. 



The adnate condition of the hydrotheca aff"ords, in fact, no di- 

 stinctive character at all ; and if Thtiiaria is to stand as a legiti- 

 mate genus, some other character must be sought for. Now this 

 will be found in the mode of division of the hydrocaulus into 

 internodes. In all the true Sertulariidans {Sertularia, Sertularella, 

 DipTiasia) there is a joint occurring at regular intervals between 

 every two or every two pairs of hydrothecae quite irrespectively 

 of the degree of adhesion of the hydrotheca to the hydrocaulus ; 

 while in Thuiaria the joints occur at distant and, for the most 

 part, irregular intervals, thus leaving numerous hydrothecse to be 

 carried on each internode. It is this, combined with the far less 

 distinctive feature of the more or less adnate condition of the 

 hydrothecse, which gives its peculiar aspect to a Thiiaria, and 

 which must be taken as the essential character of the genus*. 



Thuiaeia. 

 Thuiaria crassicaulis. Plate XVI. figs. 1-5. 

 TropJwsome. Hydrocaulus attaining a height of six inches, very 



* Guided by tbis'character, some of the species hitherto included among the 

 true Sertulariaiis must be removed to Thuiaria. Thus the Sertularia-argenfea 

 and & cupressina of authors are true Thniarirp. Indeed 6". cupresstiia, even iu 

 subordinate details, the immersion of its hvdrollieca? and its peculiar ramification, 

 is in all respects a tTpioal Thuiarian. 



