1895 THE MICIIOSCOPE. 71 



By bearing these distinctions carefully in mind the 

 student will be able very quickly to assign any of these 

 forms to the proper section, and then the determination 

 of the family and genus will occasion very little diffi- 

 culty. 



Thus in each of the sections, we have three families 

 which are separated by the presence or absence of what 

 are called radial apophyses (or outgrowths) that is, ribs, 

 wings, spines, or feet. And these three families occur 

 in the same order in each section, so that we meet with 

 these characteristics over and over again, namely, three 

 radial apophyses. Take for example, section A, in which 

 the forms have the cephalis only. The Fam. Tripocal- 

 pida has three radial apophyses, which in this family 

 are feet, and the presence of lateral ribs or ridges dis- 

 tinguishes the genus Tripocalpis from Tripilidium. The 

 Fam. Phsenocalphida has numerous radial apophyses, 

 which in this family also are feet, and the genera are 

 distinguished by having the mouth oj)en or closed and 

 by the presence or absence of radial ribs or ridges and a 

 horn. The genus Phsenocalpis may also be recognized by 

 the presence of a slender column in the axis of the shell 

 cavity. And the Fam. Crytocalpida has no radial 

 apophyses. The key to the genera here is so plain as to 

 call for no comment. 



Now these main distinctions run through all the sec- 

 tions. Thus in section B, in which the forms have two 

 joints, both cephalis and thorax, the first Fam. Tripocyr- 

 tida starts ofi" as before with three radial apophyses, 

 which in this family are either ribs or wings or feet. In 

 the first division the mouth of the thorax is open. 

 Where the ribs are said to be enclosed in the wall of the 

 thorax, what is meant is that they do not simply appear 

 on the outside of the thorax but are built into it. By a 

 latticed thorax is meant one with pores. The peris- 



