450 DR F. A. BATHER. 



has been divided by a radiating ridge. Or this may be plate 11, and the small 

 rhomb may indicate its union with a greater upward extension of plate 1 than occurs 

 in C. penniger. 



§ 347. Relations of the Species. — The Girvan specimen agrees so well with 

 Jaekel's diagnosis of C iyiterruptus, that there is no obvious reason for separating 

 them. The most important result of this is to fix the age of that species as later than 

 any other of those described from N.W. Europe. This is satisfactory, since one would 

 naturally regard the multidisjunct structure of the pectinirhombs as a late modification. 

 The correlation of this structure with an excessively large number of rhombs is tlie 

 converse of the case observed in C. constrictus, and may be explained on the same 

 principle (§ 335). Associated with this it is not surprising to find excessive modifica- 

 tion in the arrangement of the thecal plates. 



Further, it was only to be expected that so highly modified a form should be rare. 

 At present only three imperfect examples are known. It is the more remarkable that 

 these three should come from two such widely separate regions. 



§ 348. Pleurocystis. 



1854. Pleufueystites E. Billings, Canad. Joiirn., vol. 2, p. 251. 



1889. Pleuroeystites Bill., S. A. Miller, "N. Amer. Qeol.," p. 272. 



1891. Pleurocystis Bill., P. H. Carpenter, Joiirn. Linn. Soc, Zool., vol. 24, p. 12. 



1896. Pleurocystis Bill., E. Habckel, " Amph. und Cyst.," Festschr. Gegenbaur, vol. 1, p. 44. 



1899. Pleuroeystites Bill., O. Jabkel, " Stammesgesch. d. Pelmat.," p. 231. 



1900. Pleurocystis Bill., F. A. Bather, "Treatise on Zoology : Ecliinoderma," p. 64. 



§ 349. Diagnosis. — A Oheirocrinid in which the form of the theca is flattened and 

 approaches bilateral symmetry ; one face almost entirely occupied by the periproct, 

 filled with small plates, and bounded by plates, 2, 3, 8, 14, 12, 7, and sometimes by 

 small portions of 6 and 9, plate 13 is crowded out; with not more than three pectini- 

 rhombs — 1-5, 10-14, 11-12 — all on the antanal face, and all conjunct ; with only two 

 subvective grooves, each leading to a stout brachiole, longer than the height of 

 the theca. 



§ 350. Genotype. — As founded by Billings (1854) there were three geno-syntypes, 

 P. squainosa, P. Jilitexta, and P. rohusta, introduced in that order. Haeckel 

 (1896, p. 44) gives P. Jilitexta as " Species typica " ; but, so far as I can ascertain, the 

 genotype was first fixed by S. A. Miller (1889, p. 272) as P. squamosa. 



§ 351. Taxonomic History. — The references (§ 348) are confined almost entirely to 

 those that have some bearing on systematic or nomenclatoral technicalities, and, except 

 for the formal change of termination, it may be said of Pleurocystis that it is happy in 

 having no history. The reasons for this are, first, the extreme modification of structure, 

 which forbids confusion with any other genus ; secondly, the difficulty of distinguish- 

 ing the various species and varieties, and the consequent absence of temptation to 

 separate them into subgenera. 



