22 RHIZOPODA. 



which is last formed is termed the ultimate seg- 

 ment. Each of the segments, viewed externally, 

 is said to have two margins, the anterior, which is 

 nearest the ultimate segment, and the posterior, 

 which is nearest the primordial one. The parti- 

 tions which separate the contiguous segments 

 from one another are termed septa, each of which 

 is perforated by one or more septal apertures, and 

 in most cases indicated externally by a ridge or 

 depression, called the septal line. The superficial 

 area of each septum, corresponding with the en- 

 tire breadth of that portion of the shell where it 

 occurs, has been designated the septal plane. 



In the nautiloid forms the term spiral suture 

 is employed to denote the line by which each 

 convolution is separated from that on either side 

 of it. Here the entire shell is said to have two 

 lateral surfaces, and a peripheral margin. The 

 shape of the latter, which varies considerably in 

 different forms, determines also that of the septal 

 planes. Each of these last forms three angles, 

 the peripheral and the two umbilical angles, the 

 latter being so termed because " directed towards 

 the centre of each lateral surface occupied by the 

 primordial segment, where there is usually a de- 

 pression or unibilicus.^^ Each of the segments 

 also has three margins, an anterior, a posterior, 

 and a peripheral. It has likewise two angles, an 

 anterior umbilical and a posterior umbilical. 



In the trochoid shells the surface on which the 

 primordial segment appears is termed the " poste- 

 rior," while the opposite extremity is known as the 

 inferior, latei^al surface. 



Among the straight types, such as Nodosaria, 

 the term " lateral aspect " is applied to the shell. 



