188(1.] NATURAL SCIENCES OF PHILADELPHIA. 121 



composed of single quadrangular joints and destitute of pin- 

 nules ; the ventral grooves lined by alternate plates. Form of 

 the anus, ventral covering and length of arms unknown. 



Remarks. In our analysis, we have taken the absence of the 

 right posterior radial to be a fixed character. It is, however, 

 possible that this plate was exceptionally undeveloped in the 

 t^-pe specimen, and in this case Baerocrinus Ungerni^ the only 

 known species of the genus, must be placed under Hoplocrinus. 



18fi4. Baerocrinus Ungerni Volborth, Bull, de I'Acad. des Sci. de St. Petersburg of 

 1865, tome viii, p. 178, with plate (adv. sheet), November, 1864, p. :i7. 

 Eiehwaid, 1859, described and figured under the name of Homocr. dipentas 

 Lethieca Ross., v, p. 183; and 1865, Bull, de Moscou, iii, p. 160. Volborth, 

 1865, Baerocr. Ungerni, Bull, de Moscou, iv, p. 442. Eiehwaid, 1866, 

 Homocr. dipentas, Bull, de Moscou, i, pp. 149-161, PI. 8, fig. h. Grevingk, 

 1867, Baerocr. Ungerni, Ueber Hoplocr. dipentas and Baerocr. Ungerni, 

 with plate; also Arch. f. Naturk. Liv. Ehst. and Kurlands, Ser. I, vol. iv, p. 

 110. Schmidt, 1874, Hybocrinus dipentas, Memoires de I'Acad. Imp. des 

 Sci. de St. Petersb., Ser. vii, tome xxi. No. 11, p. .3, PI. 1, Figs. 1-2. P. 

 Herb. Carpenter, 1882, Baerocr. Ungerni, Quart. Jour. Geol. See. London 

 (August), pp. 298-312. W. and Sp., 1883, Baerocr. Ungerni, Amer. Jour. 

 Sci., vol. xxvi, November, p. 365. Brandschiefer, Lower Silurian. Erras, 

 Kussia. 



HOPLOCRINUS Grevingk. 



1867. Grevingk, XJehev Hoplocr. dipentas a,nd Baerocr. Ungerni, p. 9, 

 1867. Greviogk, Arch. f. Naturk. Liev.-Ehst. und Kurlands, Ser. I, vol. 



iv, p. 110. 

 1883. W. and Sp., Anier. Jour. Sci., vol. xxvi, November, p. 365. 



Syn. Apiocrinus, 1843, Leuchtenberg ; 1856, Eiehwaid. Ilomo- 

 crinus, 1859, ^ichw&ld.Eybocrinus, 1864, Volborth ; 1874, F. 

 Schmidt ; 1879, Zittel ; 1882, Carpenter. 



Volborth, F. Schmidt, Zittel and Carpenter have made Hoplo- 

 crinus a S3'nonym of Hybocrinus, although they admit that the 

 two forms differ in the construction of their azj'gous side. We 

 have heretofore directed attention to the fact that the differences 

 in the number and arrangement of the azygous and anal plates 

 have been generall}^ regarded as good characters for distinguish- 

 ing genera, and especially among the Cyathocrinidde and allied 

 families in which those plates frequently form the only means 

 for generic separation. That the one azygous plate in Hoplo- 

 crinus is equivalent to the two in Hybocrinus^ which we have 

 fully admitted, does not warrant an exception in this case, as the 

 differences in the arrangement of the respective plates are 

 evidently the result of modifications from one to another. We, 

 therefore, accept Grevingk's genus, and give the following . 



