:885.] NEW-YORK MICROSCOPICAL SOCIETY. 145 



TRICERATIUM DAVYANUM. 



BY P. H. DUDLEY, C. E. 

 {Read Afay \e^th, 1%%S-) 



The following description of Triceraiiujti Davyatium, Grev.,' 

 was published by Dr. R. K. Greville in the " Quarterly Journal 

 of Microscopical Science" in 1862 (See Vol. II., New Series, p. 

 232 ; and PI. X., Fig. 4) : " Valve with slightly convex sides, 

 rounded angles, and large punctate pseudo-nodules ; border and 

 the central triangular space largely cellulate, the former divided 

 into compartments by transverse lines. Distance between 

 the angles, .0068 in. to .0080 in. There can be no question that, 

 for beauty and interest, this magnificent species stands at the 

 very head of the genus, distinguished, as it is, for many fine 

 forms. The only two examples as yet known were both dis- 

 covered by my acute and indefatigable friend, Mr. Johnson." 



I have taken, in different focal planes, some photographs of 

 side-views of the specimen of Triceratiiim Davyanum mounted 

 by Mr. Febiger which was exhibited by our President at the 

 meeting of May ist. These will enable us to some degree to 

 understand its sculpture without resorting to the expedient of 

 making sections of a diatom which is so rare and so highly 

 prized. They are direct reproductions, in printer's ink, of the 

 negatives, and exhibit exactly the relation of all the parts as 

 given by the microscope. They will be seen to furnish several 

 important features not shown in Dr. Greville's engraving. They 

 were taken with a lens of high power and high angle (N. A., 1.13), 

 a homogeneous-immersion iVth, my object being to limit the 

 penetration so as not to show the entire convexity in one view, 

 and so as to afford, besides, a basis of measurement. The mag- 

 nification is four hundred and eight, linear. 



In photograph No. i is shown the general appearance of the 

 specimen when the markings in the pseudo-nodules, at the 

 angles of the diatom, are in focus, the rim of those nodules 

 rising apparently ii^;5?i5^th of an inch above the markings. In this 



* " I have great pleasure," said Dr. Greville, " in dedicaling this rare species to 

 Dr. Davy, who collected aud brought home the material in which it was discovered." 

 The material was from the Barbadoes deposit. 



