CARADOCIAN CYSTIDEA FROM GIRVAN. 507 



and American species of Cheirocrinus older than Pleurocystis have not been discovered. 

 The gap between the Trenton Limestone and the Russian Vaginatenkalk therefore 

 provides a convenient field for the exercise of imagination, 



5 599. But, if Cheirocrinus be considered more closely, difficulties of another character 

 are brought out. The invagination of the base, so common in the genus, and the 

 approximately equal development of the brachioles on all sides are signs of a normal 

 pelmatozoic attitude. The distribution of the pectinirhombs may not be fully explained 

 by my hypothesis of pressure from the gut, but at least it shows no obvious signs 

 of having been influenced by any departure from the erect position. What, then, 

 are the "marked modifications pointing to the assumption of a prostrate habit" 

 (Kirk, 1911, p. 17)? 



§ 600. The sole relevant character appears to be the enlargement of the periproct. 

 But this needs more careful examination. It is particularly unfortunate that the 

 position of the actual vent is not known in any species. The shape of the periproct, 

 however, is suggestive. If the theca be placed with the periproct towards the observer, 

 then the long axis of the periproct is seen to stretch upwards from left to right. It is 

 natural to suppose that this indicates the course of the rectum, and that the vent was 

 at one end or other of the elliptical opening. Which end ? If at the lower end on the 

 left, then it was indeed passing downwards, as in Pleurocystis, but towards the other 

 side of the theca ; if at the upper end, then the direction of movement was even more 

 opposed to that in Pleurocystis. Pleurocystis aside, the latter view seems preferable, 

 for it is more in accordance with the pelmatozoic position, and is easier to reconcile 

 with the normal coil of the gut. 



§ 601. Another difference from Pleurocystis is presented by the fact that in fossils of 

 Cheirocrinus the periproct is so frequently found lying to the side. This, which was 

 first brought forcibly to my notice in C. constrictus (§ 321), is well shown in C. insignis 

 (Jaekel, 1899, pi. 18, f. 2), C. granulatus (op. cit., pi. 11, f. 4), C. Walcotti {loc. cit, f. 9), 

 apparently in C. atavus (loc. cit., f. 5), C. interruptus {op. cit, pi. 10, f. 9). The fact, if it 

 proves nothing more, at least shows that the extension of the periproct caused no such 

 flattening of the theca as would induce it to fall on one of the flattened faces rather 

 than on any other face. No evidence to the contrary is afforded by the figures of 

 either E. Billings (1858) or F. Schmidt (1874). 



§602, Here also let us recall the rarity in Cheirocrinus of pectinirhomb 11-12, 

 one of the three that characterise Pleurocystis. 



§ 603. For all these reasons we must conclude that, although Pleurocystis may 

 have been derived, by stages at present unknown, from an early form of Cheirocrinus, 

 still the genus Cheirocrinus, as a whole, pursued a different path of evolution, and is 

 by no means a mere undeveloped Pleurocystis. It has just as good a claim to be 

 considered an ancestor of the symmetrical Cystohlastus as of the compressed Pleuro- 

 cystis, and {pace Dr Kirk, 1911, p. 17) it may also have given rise, through 

 Glyptocystis, to the Callocystidae. 



