1881.] NATURAL SCIENCES OF PHILADELPHIA. 331 



the iuterradial and interbrachial spaces, natural ridges, which 

 at the inner side constitute grooves or open galleries for the 

 receiDtion of the ambulacral tubes. It is a remarkable feature of 

 this genus, that the inner floor within the grooves is lined with 

 indentations, which evidently formed passages through the vault, 

 perhaps in connection with the hydrospires, and either communi- 

 cated with the surrounding element or with soft appendages. 

 These indentations have been observed only in Physetocrinus and 

 Strotocrinus (not Teleiocrinus], but in several species, and hence 

 may be considered a good generic distinction. They are located 

 between the sutures along the middle series of plates, and are 

 best observed in natural casts (PI. 19, fig. 5), in which they form 

 little tubercles, which extend to the arm bases. 



Anus in form of a circular opening through the vault, surrounded 

 by very minute plates, which were somewhat flexible, for they bulge 

 out frequently into a small protuberance. 



Arms long, the sides somewhat angular ; constructed of two 

 series of pieces; pinnules composed of slender joints (without 

 hooks), closely arranged. 



Column like that of Actinocrinus. 



Geological Position^ etc. — Physetocrinus has been discovered 

 only in America, and in rocks of the age of the Burlington lime- 

 stone. 



We recognize the following species : — 



1S69. Physetocrinus asper Meek and Worthen. (Strotocr. — Physetoor. — asper) 

 Proc. Acad. Nat. Sci. Phila., p. 161 ; also Geol. Rep. 111., v, p. 351, PI. 7, fig. 

 1. Upper Burlington limest. Burlington, Iowa. 

 1869. Physetoor. dilatatus Meek. (Strotocr. — Physetocr. — dilatatus) Proc Acad. 

 Nat. Sci. Phila., p. 162; also Geal. Rep. 111., v, p. 36:!, PI. 10, fig. 6. Upper 

 Burlington limest. Burlington, Iowa. 

 1S5S. Physetocr. ornatus Hall. (Actinocr. ornatus) Geol. Rep. Iowa, i, pt. ii, p. 

 583, PI. 10, fig. 12; Meek and Worthen, 1873, Geol. Rep. III., v, p. 349, 

 Lower Burlington limest. Burlington, Io»a. 

 Syn. Actinocr. senarius Hall, 1860. Supp. Geol. Rep. Iowa, p. 25. 

 1858. Physetocr. ventricosus Hall. Type of the genus. (Actinocr. ventricosus) 

 Geol. Rep. Iowa, i, pt. ii, p. 595, PI. 11, figs. 6 a, b ; Meek and Worthen, 

 Geol. Rep. 111., v, p. 349. Upper Burlington limest. Burlington, Iowa. 

 Si/n. Actinocr. ventricosus var. internodius Hall, 1861. Desc. New Sp. 



Crin., p. 3; also Bost. Journ. Na'. Hist., p. 278. 

 Syn. Actinocr, var. reticulatus 1861. Desc. New Sp. Crin., p. 3. 

 Syn. Actinocr. var. cancellatus. Bost. Journ. Nat. Hist., p. 279. 

 S.n. Actinocr. subventricosus McChesney, 1860. New Pal. Foss., p. 21; 

 also 1S69, Chicago Acad. S;i., i, i)t. i, p. 16, PI. 4, fig. 6; Meek and Worthen, 

 Geol, Rep. 111., v, p. 349, 



