DKsciiii'Tioxs of tiii; si'i;i n:s. 217 



columnar cavity; section nearly round; periphery almost equatorial. Basal plates 



small, entirely hidden within the columnar cavity, and forming a small inverted cup. 

 Radial plates very short with scarcely any subdivision into bodies and limbs, their 

 distal ends bent inwards; basiradial sutures within the columnar cavity; lips pro- 

 minent, and horizontally projecting, each with a backwardly directed ridge running to 

 the edge of the columnar cavity, within which the ridges are continued, two being less 

 prominent than the other three. Deltoid plates occupying two thirds the length of 

 the calyx, stave-shaped, central portion of each plate raised a little ahove the lateral 

 parts ; anal plate more prominent than the central portions of the four normal deltoids, 

 and separate from the lateral portions of theazygos one, horizontally truncated at the 

 proximal end, in some specimens projecting above the summit, in others depressed 

 below it; radio-deltoid sinuses narrow, and as long as the calyx, but the radial portions 

 remarkably short. Ambulacra very long and narrow, with parallel sides, and projecting 

 above the edges of the sinuses ; side plates quadrangular, at least CO on each side of an 

 ambulacrum; outer side plates oblong, or tear-shaped ; hydrospire-pores very large; 

 lancet-plate exposed only in the ambulacral groove. Hydrospire-folds two ; the sacs 

 pyriform. Spiracles oval, opening obliquely upwards and inwards towards one another 

 in pairs, their outer margins elevated and rim-like. Mouth small, covered by a variable 

 number of summit-plates arranged round a central one. Anus oval, opening more 

 or less in two planes, the proximal portion being oblique to the summit, and the 

 distal half of the opening horizontal, with a prominent margin. Column unknown. 

 The lateral portions of each deltoid plate are cancellated, the central portions granu- 

 lose, with occasional wavy chevron-like undulations corresponding to the outliue of 

 the radio-deltoid suture ; the radial plates have similar markings. 



Remarks. We have experienced no little difficulty in deciphering the mutual 

 relations of the various parts of the calyx in Eloeacrinus Vemeuili — the "petrified 

 hickory nut" of Lyon. This is due to the highly silicified nature of most of our 

 specimens, but we believe our explanation to be substantially correct. 



The typical form of this species, as figured by Lyon, is olive-shaped, but it passes 

 through an almost endless series of insensible gradations of outline up to the marked 

 variety which we shall presently describe under the name of Elceacri/nts Vemeuili, 

 var. pomum. Troost proposed another species, Olivanites (jlobosus l , but both 

 Eoemer 2 and Shumard 3 refer this to E. Vemeuili as a synonym. The latter author 

 also places here the Pentremites carioides, D. D. Owen; but we have not had access 

 to the work 4 in which this is described ("? or mentioned only). 



Little difficulty will be experienced in distinguishing between E. Vemeuili on the 



1 Troe. American Assoc. Adv. Sci. for 1849 [1850], p. 62. 



2 lironn's Lethsea Geogn., Dritte Aufl. 1852-54, Tlnil ii. p. 284. 

 » Trans. St. Louis Acad. Sci. 1865, vol. ii. no. 2, p. 369. 



4 Cat. GeoL Specimens Ohio Valley, 1843. 



2p 



