194' THE AMERICAN MONTHLY [June 



(dry mount), nor in the Navicula firma var. hitchcockii 

 (balsam), to be mentioned presently, although this last 

 is a striking object with an oil immersion |. With regard 

 to the different forms of nodules they may be divided in- 

 to four groups. 1st, the Maryland forms (fig. 3 and 4). 

 2ndly, the angulatum form (fig. 6). 3rdly,the balticum 

 form, which outwardly resembles the angulatum form, 

 but its raphae pipes enter farther into the nodule, and 

 they have distinct knobs where they turn down into the 

 thickness of the silex, also the perforated structure is 

 displaced to the right and left of the nodule. 4th and 

 lastly, we find a nodule intermediate between the angula- 

 tum and balticum forms. 



In the first group we have the Maryland Pleurosigma, 

 affine, affine var. fossilis, rigidum, rhombeum, convexum, 

 naviculaceum. 



In the second P. angulatum, formosum, decorum, elong- 

 atum, strigosum, quadratum, aestuarii, acuminatum, 

 littorale, fasciola. 



In the third various varieties of P. balticum. 



In the fourth P. attenuatum, strigilis, hippocampus, 

 eximum, scalproides. 



In hippocampus the ends of the raphae pipes are equally 

 turned aside, and being strong are very easy to observe ; 

 there are also small knobs like balticum. 



Among the Naviculacese there are in Moller's Type 

 Plate six varieties of N. firma, which have the extremi- 

 ties of their raphae pipes turning in opposite directions. 

 That of N. firma var. latissima, now known as Nav. 

 tumescens, was admirably figured by Mr. Karop and de- 

 scribed by me. The diatom there is called a Pinnularia, 

 but I have since been informed that it is N. tumescens. 

 These pipes come down the raphae 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 



