72 THE MICROSCOPE. May., 



tome is the circle formed by the mouth of the shell. 

 Where the feet or spines are said to be solid, what is 

 meant is that they have no pores. Wings are only 

 broadened spines and in several of the genera they are 

 so slightly broadened that they are just stout spines and 

 nothing more. But standing out as they do from the 

 sides of the shell, they are called wings because of their 

 position. In the Fam. Anthocyrtida again, there are 

 numerous radial apophyses as was the case with the sec- 

 ond family of section A. The only thing in the key to 

 this family that needs to be explained is that when the 

 meshes are said to be simple, the meaning is that the 

 spaces between the meshes are open. Fenestrated 

 meshes are those in which the spaces are partially filled 

 by a fine net-work. And in the Fam. Sethocyrtida we 

 have no radial apophyses, as was the case with the third 

 family of section A. 



Again, in section C, in which the forms have three 

 joints, cephalis, thorax and abdomen, we have exactly 

 the same order for the families, Podocyrtida having 

 three radial apophyses, Phormocyrtida numerous radial 

 apophyses, and Theocyrtida none. The abdomen is the 

 third or lowest joint. Remembering that horns are 

 spines on the top of the shell, (see Figs. 29-32) wings, 

 sjjines on the sides (see Fig. 32) and feet, spines on the 

 bottom (see Figs. 29-31), the key to the genera will be 

 readily understood, the only other point needing expla- 

 nation being that simple feet are those which as in Figs. 

 29-31 are not forked or branched. When the feet are 

 divided or forked they are said to be ramified. What 

 the student needs to note carefully in these families is 

 whether the wings or ribs are on the thorax, or partly 

 on the thorax and partly on the abdomen or only on the 

 abdomen. Horns of course will only be at the top of 

 the shell and feet at the bottom. 



