THE &MBT7LAGBA. C3 



E. The Pinnules. 



The lateral appendages of the ambulacra of a Blastoid which arc generally known 

 as " pinnules" were discovered hy Roemer 1 in the year ISIS in the genus Penfremttes, 

 and have since been seen in Troostocrmus (PI. HI. fig. 13), Gfranatocrinus, Orqpho- 



crimis, Stepkanocriwus, and other types, though only in specimens from American 

 formations. No trace of them has yet been discovered in any European Blastoid. 

 We have nothing to add to Roemer's account of their structure, except that the 

 arrangement of the lower joints does not appear to us to be always quite so regular 

 as he describes. In our specimens (PL XL figs. 10, 17) there may be some two or 

 three wide basal joints before the double row commences, instead of only one as stated 

 by Roemer ; while we very much doubt whether in some of the pinnules the joints 

 are not single throughout. 



Roemer discovered the pinnules before the relations of the hydrospire-pores were 

 properly understood, and he imagined that the pinnules were attached over the pores, 

 which served to transmit their nutritive canals. But subsequent investigations have 

 not confirmed this idea. Billings 2 stated distinctly that the pinnule-socket is situated 

 between two of the pores, and Hambach 3 described it in the same way. Our own 

 observations have led us to the same conclusion, which is the one most probable, 

 theoretically, according to our present knowledge of the relation between the pores 

 and the hydrospires. 



Shumard 4 has described the summit of Pentremites sulcatus as being occasionally 

 covered by a little pyramid which consists of about fifty pieces arranged in ten series, 

 and Hambach 5 refers to it as made up of little tubes. We have not seen this 

 structure in Pentremites sulcatus, but from the condition of the specimen figured in 

 PL V. fig. 28 we should be inclined to infer that the pyramid in question is composed 

 of the proximal pinnules immediately round the peristome, which appear to be 

 different from those more distally placed, as is so often the case in the Crinoids. 



F. The Covering Plates of the Ambulacra. 



It was announced by Dr. C. A. White 6 in 1863 that the ambulacral grooves of 

 Orophocrinus stelliformis are " neatly filled by a compound series of minute plates, 

 which closely connect at the summit with five small plates " covering the peristome ; 



1 Xeues Jahrb. f. Mineral. 1848, pp. 292-296. 



3 Amer. Journ. Sci. 1870, vol. 1. p. 228; Ann. & Mag. Nat. Hist. 1871, vol. vii. p. 145. 



3 Trans. St. Louis Acad. Sci. 1880, vol. iv. no. 1. p. 151. 



1 Ibid. 1858, vol. i. no. 2, p. 244. 



5 Ibid. 1884, vol. iv. no. :i, p. 54:5. 



6 Boston Journ. Nat. 1 list . 1863, vol. vii. no. 4, p. 1^7. 



