456 DR F- A. BATHER 



naturally be that towards the antanal face, viz. the anterior ; but if the antanal face 

 was downwards, as seems quite possible (§ 608), then the brachiole would be that 

 towards the anal face, viz. the right anterior. This latter was the homology adopted 

 by me in 1900 (p. 64). 



The structure of the Brachioles has been described and figured by Dr Jaekel 

 (1899, p. 232) and myself (1900, p. 65, f. 34). 



§ 375. The three Pectinirhombs have been described by Dr Jaekel (1899, p. 232). 

 He alludes to their conjunct structure as primitive, in contrast to their reduced number, 

 which is specialised : the two features are probably correlated (see § 335). In a few 

 cases, however, the folds in one half of a rhomb may be closed over (§§ 494, 509, 528, 

 543 ; text-figs. 67, 71-75). Dr Jaekel also points out that the two adoral rhombs 

 only unite plates of Circlet III. This contrasts with the Echinoencrinidae and 

 Callocystidae, in which the rhombs only unite Circlet III. with Circlet IV. The 

 Pleurocystine rhombs 11-12 and 10-14 both occur in Glyptocystis, and 10-14 is 

 probably present in all species of Cheirocrinus ; 11-12, however, appears to be rare in 

 Cheirocrinus, and the species from which it is absent cannot be in the ancestral line of 

 Pleurocystis. 



§ 376. The position of the pectinirhombs in relation to the shape of the theca is 

 also noteworthy. Though confined to the antanal face, they seem to get as near as may 

 be to the margins, leaving a clear space in the middle. They are generally surrounded 

 by a thickened border, and an angle of this coincides with the umbo on plates 10 and 

 11, but is a little below the umbo on plate 5. These three umbones project, so that the 

 clear space between them is depressed, and so that the theca, if placed with its antanal 

 face on a smooth surface, rests on these three points, perhaps with occasional support 

 from the umbones of plates 9 and 6. It is from these five umbones that the strengthen- 

 ing axial folds spread over the antanal face (§ 402), and these facts are not without 

 bearing on the position of the theca with reference to the sea-floor (§ 608). 



§ 377. Though the number of the pectinirhombs is normally three, still in 

 P. Rugeri and P. procera there is no trace of rhomb 1-5, and in P. anglica it is 

 greatly reduced. 



§ 378. The pectinirhombs vary in shape, and appear to furnish good specific char- 

 acters. They may be elongate either along the suture, as in P. filitexta (text-fig. 68), 

 or across the suture, as in P. Rugeri (text-fig. 69), with all intermediate propor- 

 tions. This naturally involves a diff"erence in the number of folds. Dr Jaekel gives 

 the approximate number of folds as from 7 to 12 in rhomb 1-5 ; from 14 to 24 in 

 rhomb 11-12; from 21 to 36 in rhomb 10-14. These numbers are probably based 

 on the Canadian species ; if the British species be considered, there is far wider 

 variation. Thus in P. anglica there are at most 12 folds in rhomb 10-14 (text- 

 fig. 70), in P. Ruge7'i not more than 9 in that rhomb (text-fig. 69), but about 11 in 

 rhomb 11-12. On the other hand, in P. gihha there may be no less than 20 folds 

 in rhomb 1-5, and 50 in rhomb 10-14 (text-fig. 71). The ratio in that species is 



