284 PALAEONTOLOGY 



to cross from one side of the sicula to the other. This 

 is called the crossing canal, and the side on which it lies 

 is called the reverse, while the side on which the sicula is 

 in front is the obverse. After thecae have thus been 

 formed for each stipe, further thecae are developed alike 

 on each. Each theca is straight, and being at an acute 

 angle to the direction of the stipe, overlaps the adjacent 

 theca considerably. If tangents are drawn to the outer 

 face of the first theca on each side, the angle enclosed by 

 them is the angle of divergence, which in this species is less 

 than 45. This angle is not maintained by the stipes, as 

 lower down they become sub-parallel : a polypary so dis- 

 posed is described as dependent. Seven British species of 

 Didymograptus have this dependent form : they are dis- 

 tinguished from one another by differences of detail in 

 the thecae. 



There are other species which begin their growth like 

 D. murchisoni and form two stipes, but with a different 

 angle of divergence or different final direction. 



Thus if the angle of divergence is about 90 and the 

 stipes continue at that angle, the polypary is said to be 

 declined ; if starting at less than 90 they gradually 

 spread out almost horizontally, deflexed; if with an 

 angle of divergence less than 180, they quickly be- 

 come horizontal; if curved upwards, reclined; if curved 

 upwards and then downwards, rejlexed (Fig. 85). Even 

 in these last two cases (known only in one species each) 

 the original angle of divergence is less than 180. The 

 constancy of these angles and curvatures, as well as 

 the frequent abrupt bends in stipes that have been 



