CARADOCIAN CYSTIDEA FROM GIRVAN. 509 



of adaptincy itself to the varying necessities of day and night, sand-storms and food- 

 streams, water aerated by the beating of surf or poor in oxygen after a season of calm ; 

 and a method which permitted the animal to retain its upright position. 



S 607. But though Cheirocrinus kept the pelmatozoic attitude as a rule, the 

 evidence of the stem suggests eleutherozoic tendencies. There must have been some 

 good reason for this, possibly the liability of a fixed form to be torn up by storms or 

 overwhelmed by changes along a shifting shore ; but it was this semi-vagabond life, 

 no doubt, that compassed its ultimate downfall. Some enlargement of the periproct 

 to a greater extent than usual, some change of conditions that enforced its prolonged 

 dilatation, these involved a loss of equilibrium which the unfixed stem was powerless 

 to counteract. And thus started the new line that ended in Pleurocystis. 



§ 608. Nobody doubts that Pleurocystis lived a potentially free-moving life, and 

 that it lay flat on the sea -floor. But whether it lay with the periproctal face upwards 

 or downwards is a question that does not seem to have been answered — or even asked. 

 Whether everyone has assumed, as I always did, that the antanal face was uppermost, 

 I cannot say. But there are reasons for raising the question. If the change from 

 Cheirocrinus to Pleurocystis really took place in the way just imagined, then, if the 

 theca fell prone on the sea-floor, how could the anus have acted if it were undermost, 

 or what advantage would there be in a distensile region with no room for distension ? 

 It may be replied : " How could the pectinirhombs have acted if the antanal face had 

 been undermost ? " That, however, presents no difficulty, for if a Pleurocystis be 

 placed with its antanal face on a flat surface, it is seen to be raised on the umbones of 

 plates 5, 10, and H (§ 376), so that the pectinirhombs are not obstructed, and at the 

 same time the anus is in a natural sanitary position, the periproct can expand, and the 

 brachioles can move freely. Or take the argument from analogy. The animals most 

 like Pleurocystis are the Anomalocystidae ; from Trochocystis down to Placocystis it 

 is generally admitted that the uppermost face is that with the smaller and more 

 numerous plates. 



§ 609. There seems to be no argument to set against all those in favour of an upper 

 position for the anal face, unless it be the argument from mode of occurrence. In the 

 Trenton Limestone the majority of the specimens appear to be preserved with the 

 antanal face exposed, i.e. uppermost; but there certainly are numerous exceptions. 

 The evidence of the British specimens is not clear, owing to the difi"erent nature of the 

 matrix. If the position of the fossil with reference to the bedding has not been noted 

 by the collector, it is impossible to infer it from internal evidence in the case of these 

 sandy rocks. One or two facts may, however, be noted. In P. Rugeri, specimen 

 7434 (§ 448) shows four individuals all lying in the same general direction and on the 

 same side. In P. quadrata, on the other hand, Gl90 comprises two individuals lying 

 on difi"erent faces, and in 0174 either the same is the case or a single theca has been 

 broken and bent over (§ 517). 



From the facts of preservation, then, no secure conclusion can be drawn. Even if 



