E. M. NELSON ON THE NODULES IN PLEUEOSIGM^. 165 



In the second P. angulatum, formosum, decorum, elongatmn, 

 strigosicm, quadratum, cestuarii, acuminatum, littorah, fasciola. 



In the third various varieties of P. Balticum. 



In the fourth P. attenuafum, strigilis, hippocam^ms, eximum, 

 scalptroides. 



In hippocamjms the ends of the rapha3 pipes are equally turned 

 aside, and being strong are very easy to observe ; there are also 

 small knobs like Balticum. 



Among the Naviculacece there are in Holler's Type Plate six 

 varieties of N.firma, which have the extremities of their raphae 

 pipes turning in opposite directions. That of N. firma var. 

 latissima, now known as Xav. tumescens, was admirably figured 

 by Mr. Karop (Journ. Q. M. C, PI. II., Figs. 19 and 20, Vol. 4 

 ser. 2, 1891), and described by me on page 318. (The diatom there 

 is called a Piiinidaria, but I have since been informed that it is 

 N. tumescens). The pipes come down the raphse to the nodule, 

 and then they spirally descend, with a right and left hand twist 

 respectively, through the thickness of the silex into a chamber, 

 which opens into the inside of the valve by the aperture, seen 

 in fig. 19 {torn. cit.). The smallest of the six forms of iY. Jirma 

 examined is named var. Hitschcockii, and it has the most 

 diverging pipe ends of all : so much is this the case that, instead 

 of finding their way into the central nodule, they pass outside it 

 into a primary areolation on either side of the nodule (tig. 9). 

 Now this diatom, as well as the 3\ tumescens, are fossils, so it 

 would appear that these Naviculacece had put on this peculiar 

 adaptation, while the Nottingham Pleurosigmce were only 

 thinking about it. 



These observed facts naturally give rise to the following 

 questions. It is obvious that the Maryland' Pleurosigma is a 

 very old foim, so also is the P. affine var. fossilis ; as these have 

 straight or nearly straight raphse pipes, and also nodules of the 

 first group, may not these structures be taken as indications of 

 early types ? If this is the case, may we not conclude that the 

 varieties of Pleurosigmce named in the first group are survivals 

 of this old type, and may not those mentioned in the second 

 group be later forms ? Thus, for example, may we not considei- 

 P. rigidum as the most perfect survival of the oldest type of 

 Pleurosigma yet known, because it has the same form of nodule 

 and straighter raphaj pipes than any of the moi-e recent forms ? 



JouRN. Q. M. C, Series II.— No. 44. 12 



