282 PALEONTOLOGY 



i. Didymograptus murchisoni (Fig. 83) is an 

 abundant graptolite in the black shales of Middle 

 Ordovician age of Abereiddy Bay, Pembrokeshire (and 

 elsewhere). Its shape is well described by the name 

 " tuning fork graptolite." Corresponding to the stem of 

 the fork is an acutely conical body (crushed flat in the 

 shale) called the sicula, its apex drawn out into a short 

 thread called the nema. There is reason to believe that by 



FlG. 83. DlDYMOGRAPTDS MURCHISONI (BECK), UPPER 



LLANVIRNIAN. 

 (Natural size.) (After Elles and Wood.) 



S, Sicula. 



the nema the graptolite was attached to floating seaweed, 

 from which it hung downwards, and we will describe the 

 fossil as if in this position. In this description many state 

 ments. cannot be fully verified on ordinary specimens, the 

 facts having been demonstrated by Dr. Holm and other 

 Swedish palaeontologists on specimens preserved un- 

 crushed in limestone. 



The sicula probably lodged the original polyp, from 

 which other polyps afterwards were formed by a process 



