288 PALEONTOLOGY 



Phyllogfaptns (Fig. 85), which in a purely morphological 

 classification has been made into a separate family or 

 even sub-order, since it has four rows of thecse on one 

 stipe, while all other graptolites have two or one. It 

 consists of species closely allied to the reclined Tetra- 

 grapti, but with the four stipes confluent. 



2. Climacograptus wilsoni (Fig. 86) is a graptolite 

 which occurs in such abundance at the base of the Hartfell 

 Shales in the Southern Uplands of Scotland that it has 

 been chosen as a zone-fossil. It has an unbranched stipe 

 with a double column of thecae (biserial polypary), and may 

 reach a length of six centimetres or more. When com- 

 plete, the proximal end shows the sicula, having at its 

 base a very large vesicle (a specific character, not found 

 in other Climacograpti), and passing at its apex into 

 a rod, the virgula. This answers to the nema of Didymo- 

 graptus, but serves an additional function. The first 

 theca budded from the sicula begins to grow downwards 

 as in Didymograptus, but soon makes a sharp bend and 

 grows upwards, and all subsequent thecae grow in this way. 

 Consequently the virgula comes to be imbedded between 

 the two columns of thecae and serves as a support to 

 them. In many species of Climacograptus (though ap- 

 parently not in C. ivihoni] the virgula extends for some 

 distance beyond the most distal thecae, and probably 

 served to suspend the polypary from floating weed. 



The thecae near the proximal end have a simple form, 

 but higher up they acquire a double right-angled bend, 

 which gives the polypary the appearance of a double row 

 of square bodies with narrow spaces (excavations] between. 



