THE CYSTIDEA 



59 



the right and left of it ; from their likeness to combs, these areas are 

 called pectinated rhombs, more shortly " pectinirhombs." 



We may reconstruct a form like that analysed in Fig. XXI. as a type 

 (probably ancestral) from which every known genus of this family may 

 easily be derived. In this archetype two plates (r. post, and r. ant.) 

 of the aboral circlet are fused together, so that the total number is 

 twenty-four ; numbers are attached to these in the diagram, after a plan 

 originated by Forbes. The following statements may safely be made 

 concerning this archetype, those applying equally to the whole family 

 being italicised : 8 is the double plate ; a pectinirhomb joins 1 & 5, 

 while other pectinirhombs were probably distributed as in Fig. XX. ; the 

 anus is in right posterior IR, between plates 7 and 8, and below 13 ; it is 

 protected by a valvular pyramid, surrounded by a ring of smaller plates ; 

 20-24 lie between food -grooves, and are therefore interradial ; the 



FIG. XXI. 



Arrangement of plates in the supposed archetype of the Glyptocystidae. The .attached 

 numbers are those given by Forbes. The letters at the top, read from left to right, denote : 

 left anterior interradius & radius ; left posterior interradius & radius ; posterior interradius ; 

 right posterior interradius & radius ; right anterior interradius & radius ; anterior radius. Plate 

 23 bears the hydropore and gonopore ; the notches in plates 15 to 19 represent the primitive 

 position of flvo food-grooves ; between 7, 8, and 13 lies the anus. Pectinirhombs not shown. 



hydropore, defining the posterior interradius, is in 23, and above it is 

 a semi-lunar pore of unknown function (genital or excretory) ; opposite 

 these is the unbranched anterior food-groove, passing to plate 15 ; the 

 branches of the right groove pass to 18 and 19, those of the left groove 

 to 16 and 17 ; at the end of each of the five main grooves is a facet, 

 bearing a brachiole composed of two alternating series of ossicles ("biserial"); 

 the food-grooves of both brachioles and tegmen are protected by small covering- 

 plates ; the theca is borne on a stem, which at its proximal end has low 

 columnals with a wide lumen, and which tapers distally. Modifications of 

 this type take place in the following directions : Enlargement of the 

 ring of small plates around the anus, into a region of scaly, flexible 

 integument (Cheirocrinus, Glyptocystis, Pleurocystis) ; the accentuation of 

 the relations of the five main grooves to plates 15-19, coupled with 

 the sinking of those plates between those of the third series (Cheirocrinus, 



