254 Mr. E. Billings on the Structure of 



tliey would do if that organ were the centre of the ambulacral 

 system, but to the small space (c) behind it, where there ap- 

 pears to have been situated a vesicle or some other apparatus, 

 to which all of them were united. Whatever may have been 

 the structure of this central organ, from which the five main 

 grooves radiate, it no doubt represented the annular vessel of 

 the recent Echinodermata to which Prof. Thomson alludes. 



Fig. 2 represents the structure of an Amphoracrinus from 

 the Carboniferous rocks of Ireland (precise locality and spe- 

 cies not determined). There are ten arms; the test is very 

 thick ; the ambulacral channels converge to the central point, 

 but do not quite reach it ; the mouth {mv) is about half- 

 way between the centre and the margin. In this Crinoid it is 

 perfectly impossible tliat the mouth can be the centre of the 

 radial system, because the two anterior passages, between 

 which it is situated, are for their whole length tunnelled, as it 

 were, through the substance of the plates, and only penetrate 

 downward into the interior at the central space. 



Fig. 3 is a plan of the summit of the widely known and 

 remarkable fossil Caryocrinus oniatus (Say). In this species 

 there are only three, instead of five, groups of arms. In large 

 individuals there are from twelve to twenty free arms (but 

 always arranged in the three groups), with a small pore at the 

 base of each. This pore is about the size of the ovarian pore 

 of an Echinus J and can only be seen in well-preserved and 

 clean specimens. The ambulacral grooves have not yet been 

 observed, but their course is indicated by three low rounded 

 ridges, which may be seen, in some specimens, radiating from 

 a large heptagonal plate situated at c. The mouth {mv) is 

 valvular, composed of from five to eight or ten plates, and is 

 always situated near the margin between the two anterior 

 groups of arms. With the exception of the ambulacral pores, 

 there is positively no other aperture in the summit of Caryo- 

 crinus. If it be true that the mouth of an Echinoderm must 

 be always situated in the radial centre, then Caryocrinus and 

 also nearly all the paljBOzoic genera were destitute of that 

 aperture. 



Caryocrinus is a genus which seems to form a connecting- 

 link between the Crinoidea and the Cystoidea. By examining 

 numerous well-polished sections, I find that the structure of 

 the respiratory areas is the same (in general plan) as that of 

 the genera Glyptocystites, PleurocystiteSy and Echinoencrinites^ 

 as will be shown further on. The arms are also arranged in 

 three groups, as in Sphoironites and Hemicosmites^ while the 

 mouth is valvular. On the other hand, the long cylindrical 

 column and the arrangement of the arms around the margin. 



