18S4-85.] Edinburgh Naturalists' Field Club. 243 



(II.) The valves in the PseudoraphiJiea3 and Eajahidieae j^resent 

 the same structure as those in the Cryptoraphidieaa ; but 

 the alveoli, thous^h much smaller, may be observed in 

 some genera of liaphoneis, Pleurcsigma, Sec, by the use 

 of Smith's medium. 

 (III.) The beads of Diatoms are formed by alveoli and striae, 



and often by tlie walls of the alveoli. 

 Tlie general inference arrived at by Professor Smith as to the 

 nature of the valvular sculpturing was, that it was all reducible to 

 modifications of a " cellular tissue," analogous to that which is so 

 manifest in the cases of Triceratium and Isthmia. In the conflu- 

 ence of the component " cells " of this " tissue " along certain 

 lines, he found an explanation of the appearances generally known 

 under the designation of " costas " or unresolvable stria?. The 

 longitudinal band or raphe, which has been made the basis of a 

 system of classification by Professor H. L. Smith, was regarded as a 

 strand of condensed or more solid silex, its median and terminal 

 spots or nodules being but expansions of that band of a similar 

 character, and both being designed for the purpose of giving 

 greater firmness to the frustule, so as to enable it more effectively 

 to resist the external pressure to which it might be subjected. 

 The fact that, as a result of pressure, the internal protoplasmic 

 contents never escaped at the raphes or noduli, and that when 

 fractured no disposition to break at this region was manifested, 

 were, moreover, adduced as proofs that no perforations of the valve 

 existed here. 



The genera Pinnularia, Navicula, and Gomphonema, among 

 others, may be cited as affording good examples of nodulated 

 valves ; while the genus Stauroneis is provided with a transversely 

 elongated central nodule to which the special name of " stauros " 

 has been applied. On the other hand, a simple, bare, or unorna- 

 mented area on diatomaceous valves has been designated by Smith 

 a " pseudo-nodule." 



In contrast to the views which have just been noted, it is inter- 

 esting to recall the circumstance that Ehrenberg regarded the 

 valvular puncta as in many cases real jwres; these, he believed, 

 played the part of apertures of exit for the pedal organs, 

 which might be few or many, and which were supposed by him 

 to be essential to explain the locomotion of living frustules. 

 Similarly this observer regarded the strife and costaj as in many 

 cases real Jissm^es, which were supposed to possess the double 

 function of serving at once as exit-channels for the ova and as 

 means for bringing the internal protoplasmic substance in close 

 contact with the external medium. 



Kiitzing also maintained the belief that valve-pores existed, 

 and that through them the muco-gelatinous mass so prominent in 



