CARADOCIAN CYSTIDEA FROM GIRVAN. 405 



form a hood over that end of the ellipse ; in these cases the free ends of the short u 

 have been depressed, gliding downwards over the free ends of the long U or V. 



§ 198. As seen from the interior of the theca, the ridges bound very deep cavities, 

 which present every appearance of being actual openings through the thecal wall. 

 Seen from the exterior, however, these supposed openings are generally filled to a 

 varying extent with small fragments. It must be borne in mind that, since the stereom 

 has entirely disappeared, all these appearances have to be interpreted and described 

 from squeezes in wax or gutta-percha. Had these small fragments been merely grains 

 of sand, they would be still in existence. The fact that they are represented only in 

 the squeezes shows that they were made of a material that has disappeared. Probably, 

 then, they represent some skeletal elements. Various possibilities suggested themselves, 

 but careful examination and comparison of all the available material (especially G34, 

 G52, G54, and G-57) ultimately led me to an interpretation which received its final 

 confirmation from Gl7. Here some seven of the elliptical areas are clearly preserved, 

 with traces of two or three others. The short u piece is erected to form a strongly 

 projecting hood, in the manner described above ; and it was this projection, doubtless, 

 that led to the better preservation of the structures contained within the rim. These 

 structures are here seen to be rather granular and slightly irregular plates, about eight 

 in number, disposed in alternating series so as to cover the opening and apparently 

 with a slight imbrication in the direction of the hood (text- fig. 16). I conclude, there- 

 fore, that these elliptical areas represent actual openings through the thecal wall, that 

 the long U of the opening was protected by movable cover-plates similar in appear- 

 ance and function to those of the ordinary pelmatozoan subvective system, and that the 

 short u of the opening was protected by the erection of its own bounding wall to form 

 a hood, which, on occasion, could meet the closed cover-plates and so shut in the whole 

 opening {e.g. G50). 



The proof that these structures represent the subvective system is reserved for the 

 general discussion of the habits of the animal (§§ 226-230). 



§ 199. The Vent or Anus (PI. III. fig. 38) is placed on the obverse side at the 

 top of the leg, in a position corresponding to the transverse extensions of the plates 

 on the reverse, and its presence is indicated in the better preserved specimens (G6, 

 Gil, G22, G23, G25, G26, G39, G45) by a radiating arrangement of the small plates, 

 which here become minute. These radiating lines converge to a point in about the 

 middle line of the leg, sometimes apparently below the top of the frame, at other times 

 level with it, or even rising above it, so that in the last case the plated skin of the 

 obverse side forms an extension visible from the reverse side (G22, PI, III. fig. 30 ; 

 also G23, G26, G39). It is to be inferred that the integument in this region was very 

 flexible and extensile, and that the anal opening could be protruded or retracted. 



§ 200. The Stem is attached to the thecal frame by means of marginals 4 and 5, 

 so that the suture between those two plates lies in the middle of the stem- 

 attachment. 



