300 Dr. F. A. Bather on the Homologies 



as Promaclwcrinus abyssorum. The process borne by the 

 posterior plate is indeed a developing - arm, and Clark suspects 

 " that smaller arms borne on the other interradials have been 

 lost. . . . During growth the posterior interradial arm of 

 Thaumatocrinus becomes reduplicated on all the other inter- 

 radial plates, and all of the five interradial arms gradually 

 increase to the size of the five primary arms so that the 

 10-armed Promachocrinus abyssorum results" (p. 338). It 

 follows from this that the supposed interradials of Thaumato- 

 crinus, including the posterior one or supposed anal, are of 

 precisely the same nature as the five arm-bearing plates 

 which in Promachocrinus have been added to the five normal 

 radials of the ordinary comatulid. Dr. Clark calls all these 

 plates "interradials," a term which suits his argument, but 

 which scarcely seems justified. 



In 1900 (Lankester's 'Treatise on Zoology/ iii. p. 150) I 

 suggested that these " interradial radials " were of the same 

 nature as the arm-bearing plates in the cup of the Monocyclic 

 Inadunata Calycanthocrinus and the Catillocrinidae, for which 

 plates Jaekel in 1895 had devised the excellent term " para- 

 radials." Dr. Clark now tells us how the pararadials of the 

 Promachocrinidae develop. They "arise very early in life 

 and are from the first equal in height to the radials. They 

 are probably . . . best interpreted as a sort of lateral budding 

 from, or a delayed reduplication of, the radial to the left. 

 As the radials move apart [the pararadials] continue to 

 broaden, and their development in all ways is proportionate 

 to their breadth as compared with the breadth of the normal 

 primary radials" (p. 337). The development of the arms 

 which they support bears a similar relation to the arms borne 

 by the normal radials. The opinion that each pararadial is 

 in a sense derived from the radial to the left of it is confirmed 

 by various facts. Thus the posterior pararadial always 

 maintains " a closer relation with the" left posterior radial 

 than with the right posterior (p. 336). In some thirty 

 6-rayed specimens of Promachocrinus studied by Dr. Clark 

 the supernumerary ray is in all cases but two inserted to the 

 right of the left posterior radial, and receives its food-groove 

 from the groove-trunk leading to that radial (p. 338). 



The posterior pararadial appears to originate slightly before 

 the others ; in the original specimen of Thaumatocrinus 

 renovatus its arm was more developed, and,- as just stated, it 

 is sometimes the only one to be formed. These facts are 

 very simply explained as due to the relatively greater widening 

 of the posterior interradius by the pressure of the rectum. 



