TIIK HYDB0SPIBE3 WD SPIB LCLES. I'M 



greater part of it is bridged over by the side plates and converted into the hydrospire- 

 canal (PL XVI. figa 19,20; PL XVIII. figs. 3, 5). The side plates, however, do not 

 come quite down on to flu- flattened central ends of the deltoids ; and the commence* 



ment of the radial sinus is thus left open as the spiracle, as is shown in PL I. figs. 5, 6, 

 I I, and PI. XVI. fig - . 21. Tins is also seen in the two sections of the summit which 

 arc figured on PL XI 1. In the first one (/'. pyriformis, fig. I5)the summit has I. ecu 

 ground down so far that the proximal ends of the hydrospire-slits an' exposed at the 

 sides of the deltoid pieces, except in the anal interradius. The canal within the 

 lancet-piece is seen in section in two of the rays, where a little further grinding would 

 have shown its communication with the oral ring by the radial passage beneath the 

 lancet-plate as in the other three rays. This is seen in the section of Pentremites 

 Godoni (fig. 17), which has not been ground far enough for the hydrospire-folds to 

 appear in the spiracle openings; while the two septa in the anal spiracle have never- 

 theless been almost entirely removed. 



In some forms of Pentremites the central end of the deltoid ridge is not completely 

 covered by the side plates of the adjacent ambulacra, and the spiracles then appear to 

 he more or less distinctly double. Fig. 5 on PL I. represents a specimen of Pentre- 

 mites elonqatus in this condition ; but in other examples of the same species, and in 

 /'. sulcatus, the side plates of the ambulacra are much larger, and meet one another 

 above the deltoid ridge, so as to conceal it completely (PL I. figs. 4, 10). Only five 

 openings are therefore visible round the peristome; and Say 1 made use of this cha- 

 racter in the generic name Pentremites, which, as G. B. Sowerby 2 , and subsequently 

 also Rocmer 3 , pointed out, is more correctly written Pcntatrematitcs. Cumberland ', 

 writing a few years later about the Kentucky Asterial Fossil, but in ignorance of 

 Say's description of it, spoke of the " five open perforations which probably supported 

 the arms being divided in the middle, as in many other encrinital bodies, and not at 

 all, I think, likely to be ovaries." Troost 5 , again, mentioned the presence of septa 

 in the spiracles, while Miiller 6 detected them in 1841, and further pointed out that the 

 anal spiracle was not merely the largest, as mentioned by Gilbertson 7 , but that it 

 had two septa in it which divided the lateral spiracles from the median anal opening ; 

 and their true relations were subsequently explained by Rocmer 8 . This point is 

 well brought out in the casts of Pentremites Koninckanus which are shown on 

 PL III. figs. 10-12. At each side of the impression of the lancet-plate is a thick rim 



1 American Journ. Sci. 1820, vol. ii. p. 36. 2 Zool. Journ. 1828, vol. iv. no. 13, p. 89. 



Anhiv f. Naturgesch. 1851, Jahrg. xvii. Bd. i. p. 324. ' Reliquiffi CouservatEC, 182G, p. 34. 



5 Trans. Geol. Soc. Pennsylvania, 1835, vol. i. p. 227. 



6 "Ueber den Bau des Pentaerinus Caput Medusas." Phys. Abhandl. k. Akad. Wissensch. Berlin, ls|] 

 [1843], p. 229. 



7 Zool. Journ. 1828, vol. iv. no. 13, p. 90. 



8 Archiv f. Xaturgesch. 1851, Jahrg. xvii. Bd. i. pp. 331, 332. 



