358 PROCEEDINGS OF THE ACADEMY OP [1881. 



fiimily ; Ollacrinus has lateral appendages, two to each interradial 

 area, with a central canal passing through each, communicating 

 with the inner body. The entrances of these canals into the body 

 occupy the same relative position to the ambulacral or arm open- 

 ings, as the respiratory pores to the arm openings in Batocrinus, 

 and these like those are connected with the radial grooves beneath 

 the vault. The appendages seem to be in some way connected with 

 the interradial depressions which are so frequently found upon the 

 vault of this genus and in Rhodocrinus (PI. 19, fig. 1). The depres- 

 sions are in some specimens deeper than in others, and as the 

 plates of which they are composed are smaller and thinner, it is 

 very probable that the test in these parts was flexible, liable to 

 contraction and expansion. 



Geological Position^ etc. — Nine of the thirteen genera which we 

 place among the Rhodocrinidae are restricted to the Silurian, two 

 are strictly Devonian, and the remainder which first appear in the 

 Devonian, become extinct in the lower part of the Subcarboniferous. 

 None of the genera seem to have had a great variety of species 

 or to have existed in great numbers, except some few species 

 of the Lower Silurian. 



We arrange the three sections as follows : — 



a. Gltptocrinites. 



1. Olyptocrinus Hall. 3. Beteocrintis Billings. 



2. Arch(Bocrmu8 Wachs. & Spr. 



b. Glyptasterites. 



4 Olyptaster Hall. 6. Lampterocrinus Roemer. 



Buhgen. Eucrinus Angelin. 7. Sagenocrinus Austin. 



5. Dimerocrinus Phillips. 



c. Rhodocrinites. 



8. Lyriocrinus Hall. 11. Anthemocrinus Wachs.& Spr. 



9. Ripidocrinus Beyrich. 12. Bhodocrinus Miller, 



10. Thylacocrinus Oehlert. 13. Ollacrinus Cumberland. 



P. S. — At the moment this goes to press, we received from S. A. 

 Miller a very interesting paper containing descriptions of several 

 new Crinoids from the Hudson River group, published in the 

 April number of the Journ. Cincin. Soc. Nat. Hist. Miller pro- 

 poses the name Xenocrinus for a form, which in external appear- 

 ance resembles closely Reteocrinus Billings, but from which it 



