336 THE VOYAGE OF H.M.S. CHALLENGER. 



syzygy below the fiftieth node ; but it lies along the cable by the next eleven internodes, 

 and is more or less attached to it by means of the cirri. 



The variation in the development of the basals in Pentacrinus decorus is most 

 extraordinary, and shows what little reliance is to be placed on the characters of this part 

 of the calyx for systematic purposes. In young specimens they are generally quite small 

 and inconspicuous, appearing only as little triangular knobs or rhomboidal pieces which 

 are sometimes just in contact by tlieir lateral angles (PI. XXXV. fig. 1). But they do 

 not stand out at all from the general plane of the calyx, and simjily appear as continua- 

 tions of the interradial ridges of the top of the stem upon which they rest. In older 

 and even in adult individuals they may retain this condition (PI. XXXIV. figs. 1,8; 

 PL XXXVI. fig. 3) ; while in others which are still premature they are a little more pro- 

 minent (PI. XXXV. fig. 2) ; and when a large series of specimens is examined all stages 

 are traceable between this condition and that of the individuals represented in PI. XXXVI. 

 fig. 1, and PI. XXXVII. figs. 1, 2. These have large rhomboidal basals, meeting one 

 another laterally in the re-entering angles of the calyx and standing out as prominent 

 knobs which sometimes extend beyond the interradial ridges of the stem.-' A somewhat 

 similar though less extensive variation in the development of the basals occurs in the 

 fossil Antedon scrohiculata from the Brown Jura. 



The ray-divisions of Pentacrinus decorus are exceedingly variable. I have met 

 with one case in which the number of three radials, so constant throughout the majority 

 of the Neocrinoids, is consideral )ly exceeded. The radial axillary is primitively the 

 seventh joint aliove the basal ring, i.e., it is itself a syzygial joint and there are five 

 below it, inchuling the primary or first radials. The second and third of these five 

 joints are united by bifascial articulation, just like the ordinary second radials and 

 axillaries of this species ; and in this respect therefore this aberrant form difi"ers from 

 the type represented by Metacrinus, in which the second and third joints of the ray are 

 always united by syzygy, whether the axiUary be the fifth or eighth in the series of 

 primitive radials (PI. XXXIX. fig. 1 ; PL XLVL). 



One young specimen of Pentacrinus decorus which was brought by Sir Rawson 

 Rawson from Barbados has only ten arms, as is the case in Pentacrinus naresianus ; 

 while in another dredged by the " Blake " there are only eleven, one of the primary arms 

 dividing on the fifth joint (PL XXXV. fig. 1). A thii'd individual (PL XXXV. fig. 2) 

 has two arms on each of four rays ; but on the fifth ray one of the primary arms divides 

 and one of the secondary arms so formed divides again, so that there are twelve in all. 

 Other examples again may have distichal series developed with considerable regularity 

 all round the calyx, and occasionally also one or more palmar series as weU (PL XXXVI. ; 

 PL XXXVII. fig. 2). But the total number of arms rarely exceeds twenty -five in all ; 



' In the specimen represented in PI. XXXVI. some of the first radials are considerably distorted, apparently as 

 the result of injury and subsequent regeneration. 



