293 



Pteroeides mac-andrewi, Kolliker. 

 Three young specimens represent tliis species. 



Total length of colony .... 

 Length of rachis ..... 

 Breadth of rachis .... 



Length of stalk ..... 

 Thickness of stalk .... 

 Number of pinnules .... 

 Number of rays ..... 

 Length of pro-rachidial margin of largest 

 pinnule ..... 



Breadth of insertion of largest pinnule 

 Breadth of largest pinnule . 

 Distance between pinnules . 



These specimens are specially interesting in illustration of the gradual 

 development of a peculiar type of pinnule — with the rays seeming to arise from 

 one main ray, which is also seen in P. rliombokkde, MorofF. 



The zooid-plate appears basal, but on closer inspection we find that it arises 

 at the pro-rachidial insertion of the pinnule where it is about 3 mm. high ; from 

 this point it passes upwards and departs from the line of insertion and gradually 

 tapers towards the meta-rachidial insertion, where it is reduced almost to a 

 single zooid. It is thus median and the species is included in KiJlliker's pelluci- 

 dum group. 



The mode of development of the pinnules is exemplified through different 

 stages in the more mature specimen A, but is better seen in B and C where a 

 less advanced stage has been reached. In C there is but one main ray along 

 the pro-rachidial margin. From this towards the meta-rachidial side numerous 

 smaller rays arise like the barbs from a rachis. The single main ray is very 

 broad and substantial, being composed of about five rows of rods lying abreast and 

 interlocking, no single rod extending the whole length. Between this stage and 

 the highest developed in our specimens, viz., that with five main rays, inter- 

 mediate forms with two, three, and four occur in B and A. There seems to be 

 a gradual differentiating into groups which become more robust than the others, 

 but in all cases their origin can be traced to the original single ray. In the 

 more advanced cases there are six to eight spines in each ray, but the same 

 structure as in the younger forms is traceable throughout. A noteworthy fact 

 is that between the first two rays on the pro-rachidial margin there are numerous 



1 There are iu both cases five main rays, but iu additiou to these there are numerous smaller rays 

 between the first two, counting from the pro-rachidial margin. 



