288 PALEONTOLOGY 



Phyllogmptus (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 thecae 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 (Userial 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. wilsoni) 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. 



