TROOSt's CRINOIDS OF TENNESSEE — E. WOOD. 9 



are poriferous rhombs and on one is an oval aperture [anus]. The poriferous rhombs 

 are barred (these bars are not equal in number in these rhombs; some have 16, and 14 

 at the top, and 12 below) in the direction of the smaller diagonal which gives them 

 the appearance of Venetian blinds, from which its specific name. 



The pelvis is circular with four reentering angles, composed of four [basal] plates, 

 three of which are pentagonal, and one, having its superior angle truncated, is hex- 

 agonal. The lower part of the pelvis terminates externally in a more or less projecting 

 edge and then the plates turning immediately inwards form the sides of a [deep] circular 

 excavation in which the column was inserted. 



The first [second] series of plates is composed of five; four are hexagonal, and one 

 having upon its lower margin one [i. e., one-half] of the poriferous rhombs is rendered 

 thereby irregular. They are placed in the four reentering angles of the pelvis; the 

 fifth plate is quadrilateral and rests upon the hexagonal pelvic plate, the superior of 

 which surrounding partly the oral aperture [anus] is also thereby rendered irregular. 



The second [third] series is composed of five hexagonal plates of which three are 

 rendered irregular by two poriferous rhombs and by the oral aperture [anus]. This oral 

 aperture is large and circular. One of the poriferous rhombs, on the quarter section 

 on the left of the oral aperture, occupies about one-half of an hexagonal plate; above 

 this rhomb is an elevation, running transversely and having on its summit a furrow 

 [the madreporite], and below it a single pore [the hydropore], neither of which pene- 

 trates through or deeply into the plate. The second poriferous rhomb is on the quarter 

 section on the right of the oral aperture; it occupies the upper part of the third hex- 

 agonal plate, which is thus also rendered irregular. 



The summit, which has no aperture, as in other species of this genus, may have been 

 composed of five plates. But our fossil, which is very perfect, without the least ero- 

 sion, being around the summit much complicated by the [ambulacral open] furrows 

 mentioned above, which here combine, the joints of the plates being thereby obliter- 

 ated, their number and form can not be ascertained. 



I can say nothing about the column, — the place where I found my specimen con- 

 tained numerous fragments of different crinoids. The place of insertion of the column 

 is very large in proportion to the size of the body, its diameter being 4 mm., while 

 the largest diameter of the body is 10 mm. and the length 14 mm. The column was 

 not inserted as is generally the case with crinoids. The cavity is formed of an inclined 

 slope or bevel of about 2 mm., having no articulating striae. 



I found this interesting fossil in Decatur county, Tennessee, associated with 

 Calymene Blumenbachii [=C. niagarensis], Orthoceratites, Tereb. wilsoni [=? Wilsonia 

 saffordi], etc. 



Professor Schuchert [1904, pp. 220-222] comments on this species 

 as follows: 



Remarks. — It is interesting to find that Troost had a species of Tetracystis more than 

 50 years ago, which he then referred to Echino-encrinites . It is closely related to the 

 Manlius T. chrysalis, but differs in being smaller, more circular in transverse outline, 

 and not flattened as is that species; the pectinirhombs, also, have far fewer dichopores. 

 In T.fenestratus there are 12 to 17 pores, and in T. chrysalis from 25 to 35. Of brachioles 

 the former has from 7 to 8 on each side of an ambulacrum, while the latter species has 

 11 in the same space. Plate 19 and deltoid 24 may not be present in T. fenestratus; 

 if they are, they are now obscured by the ambulacralia. However, as Troost's species 

 is otherwise closely related to T. chrysalis, it is probable that other specimens may 

 reveal these two small plates. 



Formation and locality. — The horizon in Decatur County, Tennessee, furnishing this 

 fossil, appears to be the Brownsport limestone of Foerste, formerly a part of Safford's 

 Meniscus limestone, in the upper portion of the Niagaran. 



Cat. No. 35091, U.S.N.M. 



