(library) 



THE OTTAWA NATURALIST 



Vol. XXVI. May, 1912 No. 2 



A FOSSIL STARFISH WITH AMBULACRAL 

 COVERING PLATES. 



By George H. Hudson. 

 (Plates I-III, fifteen figures.) 



Through the courtesy of Dr. Percy E. Raymond my atten- 

 tion was recently called to a very remarkable sea-star found 

 at Ottawa. Canada. Mr. J. E. Narraway, the discoverer of the 

 form, kindly allowed me to keep it through Dec, 1911, and 

 Jan., 1912, for photographic work and study. During this 

 period I made a series of forty-five different photomicrographic 

 negatives of the specimen showing the whole or portions thereof 

 under different conditions of light, angle, or mounting and at 

 various stages of a partial development which was given the 

 arms and oral cavity. Prints from fourteen of these negatives 

 were selected for use in making the figures for the plates accom- 

 panying this article. Figures 1 and 2, plate I; 2, 5 and 7, 

 plate II; and 1, 2, and 3, plate III, were made under a gum 

 dammar mounting with coverglass. For a description of this 

 process and some remarks concerning its value see "New York 

 State Museum Report 149," page 218. 



Figure 3 of plate I represents the specimen near the begin- 

 ning of my work upon it. Lines drawn down the radii reveal 

 two distinct centres. The elongation of the oral aperture along 

 the line connecting these centres, the elongation of the disc 

 itself in the same direction, the widening of the lower inter- 

 radius of the figure and the narrowing of the next interradius 

 at the right and at the left have been interpreted as indicating 

 the position of the posterior interradius and the figure has been 

 so oriented and marked. Figure 1 of this plate shows the 

 condition of the specimen at the end of such development as 

 I felt justified in making. 



Elements of the Specimen and Terminology. 

 The bordering plates will be called marginals; the large 

 single marginals of the interradii will be designated as inter- 

 radial niarginals and the remaining marginals as arm marginals. 



