1870.] BILLINGS — ON CRINOIDEA AND BLASTOIDEA. 181^ 



ward, aud resting on the first radial, whilst the lower end would 

 be upward, the tip being formed of the second radial. From 

 this it follows that the forked plates do not belong to the radial 

 but to the perisoniatic system. 



The five deltoid plates alternate with the forked plates, and are 

 also perisomatic. 



It is not certain that the lancet plates represent any of those 

 plates which in the Crinoidea arc usually called "radials." 

 They are so arranged that if they were loosened from the walls of 

 the cup, and their smaller extremities turned upward, whilst 

 their bases or larger ends retained their position, they would 

 stand in a circle around the apex, as do the arms of an ordi- 

 nary Crinoid. Their bases would alternate with the apices of 

 the deltoid plates. They would form the outside of the arms 

 whilst the grooves aud pinnula3 would be inside. Each would 

 bear, on its outer or dorsal aspect, two elongated sacks, the two 

 hydrospircs that belong to the ambulacrum. I believe that the 

 small groove in the ambulacrum of Penfremites was occupied by 

 the ovarian tube only. If this be true, and if, also, the lancet 

 l>lates represent the radial plates of the arms of the Crinoids 

 then the arm of Pentremltes w^ould have the respiratory portion 

 of the ambulacral system on its dorsal, and the ovarian portion 

 on its ventral aspect. 



In the true Crinoids, both the respiratory and ovarian tubes 

 are situated in the groove in the ventral side of the arm.-^ In 

 the Crinoids the pinnula) are attached to the radial joints of the 

 arni. In Pcntremitcs they are not connected with the lancet 

 plate, but with tlie pore plates. In P, piriformis they appear to 

 me to to stand in sockets excavated in the suture between the 

 pore plates. Midler compared them to the series of azygos 



* Thomas Say, who was the first to recogaize the Blastoidea as a 

 group distinct from the Crinoidea, also supposed the function of the 

 amhiiiacra to be respiratory. He says, " I think it highly probable that 

 the branchial apparatus ccuimunicated with the surrounded fluid 

 through the pores of the ambulacra, by means of filamentous processes ; 

 these may also have performed the office of tentacula, in couveyiu<'- 

 food to the mouth, which was, perhaps, provided with an exsertile 

 proboscis ; or may we not rather suppose that the animal fed on the 

 minute beings that abounded in the sea water, aud that it obtained them 

 in the manner of the Ascidia, by taking them in with the water. The 

 residum of digestum appears to have been rejected through the mouth." 

 (Jour. Acad. X. S. Phil., voL iv, p, 296, 1825). 



