134 Cincinnati Society of Natural History. 



unknown. (Trans. Geol. Soc. of Penn., vol. i, p. 232 as Astetias. 

 Hall. Adv. sheets 20th Rept. N. Y. State Mus. Nat. Hist., 1866 : 

 20th Rept. same 1867, p. 287.) 



Locality. — Big Harpeth River, Davidson Co., Tenn. 



Remarks. — The description given by Troost is very imperfect. 

 That given above is by Hall, who examined the type specimen and 

 redescribed it, placing it provisionally in the sub-genus Argaster. This 

 seems justified in the single row of ambulacral ossicles, the typical 

 species of Palceaster having two rows. 



Genus 2.— PETRASTER Billings, 1858. 



"This genus has both marginal and adambulacral plates, with a 

 few disc plates on the ventral side. The general form is deeply stellate 

 and the rays long and uniformly tapering. A single specimen has 

 been collected, and, as it shows the under side only, the character of 

 the dorsal surface can not be given. The structure of the mouth is 

 also unknown." "It differs from Palasterina by the presence of 

 large marginal plates outside of the disc plates." (Canadian Organic 

 Remains, decade 3, p. 79.) 



Remarks. — The figure of Petraster rigidus, the type, shows only 

 two rows of plates on each side of the ambulacral groove at the ends 

 of the rays, and three at the base. There are only about seven plates 

 in the central row near the disc, with a single plate at the junction of 

 any two arms at the disc. This plate is nearly square with the angles 

 directed toward the disc and outward between the rays. The descrip- 

 tion is very unsatisfactory and it might possibly be well to consider it 

 as a synonym of Palceaster. Prof. James Hall states* that the type 

 specimens of the genus, examined by him, are plainly referable to 

 Palceaster. He places the following species in that genus. It is here 

 kept distinct provisionally. 



1. — P. wilberanus Meek, 1861. 



Diameter in its larger part one inch, and in its smaller 0.33 inch; 

 rays slender with two ranges of plates on each side of the ambulacral 

 groove on the ventral side ; about the same size in each row, and some- 

 times alternating; all rather prominent, those in the outer range pro- 

 jecting in the form of small nodes ; about twenty-three pieces on each 



* 20th Rept. N. Y. State Mus. Nat. Hist., 1867, p. 294. 



