1885.] NATURAL SCIENCES OF PHILADELPHIA. 341 



allied to Hexacrinus. All plates of the dorsal cup, the arro plates, 

 and all interradial and summit plates, covered with numerous, 

 irregularly arranged tubercles, provided centrally with a small 

 pit for the reception of a long acicular spine. 



Basals three, large, pentagonal. Primary radials 3X5; the 

 lower one very large ; the two upper ones small. 



The anal plate has nearly form and size of the first radials, and 

 occupies a similar position. The interradials are numerous and 

 either cover the ambulacra completely, or open out to expose the 

 covering plates. All plates of the ca^-x, dorsally and ventrally, 

 except the basals, are provided with one or more movable spines, 

 also the oral plate and proximals, but not the covering pieces, 

 which, however, as stated, are not always exposed. Anus sub- 

 central. 



Arms two from each ray, simple or branching, and giving ofl 

 slender pinnules from each joint. The proximal arm plates are 

 composed of single cuneiform pieces, but these gradual!}' interlock 

 and turn into two series of alternate plates. Column round. 



Geological position, etc. — Arthroacanlha has been found in the 

 upper part of the Devonian, and of America onlj r . 



1883. Arthroacantha ithacensis Williams, Type of the genus. Amcr. Philos. Soc, 



April, p. 83, with figures. — HarailtoD gr. Near Ithaca, N. Y. 

 1882. A. punctobrachiata Williams, Trans. Amer. Phil. Soc. (April), pp. 83 and 86 

 (figured by Hall as Platycrinus punctobrachiatus). — Hamilton gr. Ontario, 

 Can. 

 *1885. A. Carpenteri (?) Hinde (Hystricrinus Carpenteri), Ann. and Mag. Nat. 

 Hist. (March), p. 162, PI. 4. — Hamilton gr. Ancona, Ontario, Can. (Prob- 

 ably a Syn. of Arthroacantha punctobrachiata Williams. 



DICHOCRINUS Minister, Rev. ii, p. 81. 



Additional species : — 



I860. D. lachrymosus Hall, Suppl. Geol. Rep. Iowa by Hall, p. 84. — Upper Burling- 

 ton limest. Burlington, Iowa. — This species was erroneous!}* referred by us 

 to Platycrinus, and was said to be synonymous with PI. subspinulosus, 

 with which it agrees in the ornamentation of the plates. Fine specimens 

 which we obtained lately, have convinced us that it is a Dichocrinus, and 

 was correctly separated by Hall. It has a comparativel} - large number of 

 interradial plates, a very conspicuous oral, and six large proximal plates. 

 The anal aperture is lateral, somewhat protruding, placed at the upper edge 

 of one of the first interradials, which is somewhat excavated. The radial 

 dome plates are composed of small alternate pieces which we followed up to 

 the second bifurcation of the raj-. Arms given off from the third secondary 

 radials, whence they branch once or twice again, always from the third plate. 



