6 PAL/EONTOLOGY 



be detected from the exterior by the appearance of a dark 

 streak : little or no trace of such can be seen externally 

 in T. intermedia. It has been found that the presence of 

 such a dark streak, indicating a strong dorsal septum, is 

 constantly associated with the presence of a long internal 

 loop, and its absence with a short loop, in all terebratu- 



FIG. 3. BRACHIAL LOOPS. 



a, T. intermedia; b, b 1 ', O, obovata (natural size); a, b, interior views of 

 dorsal valve ; 3', side view, test removed to expose the septum ; 

 H.P., hinge-plate; S., septum; So., sockets. (After Davidson.) 



loids. We may therefore divide these into a long-looped 

 and a short-looped group. 



This difference in the length of loop is regarded by 

 palaeontologists, for reasons that will appear later, as a 

 more fundamental difference than any of the others that 

 have been given. It is considered too important to be 

 expressed by a mere difference of specific name : the 

 generic (and even the family) names must also be 



