156 PROCEEDINGS OF THE ACADEMY OF 



Locality and position. Lovrer Burlington beds of Lower Carboniferous, Bur- 

 lington, Io^ya. No. 38 of Mr. Wachsmuth's collection. 



AcTiNOCRiNiTEs DELiCATcs, M. and W. 



Bod}' small, subturbinate, or widening rather rapidly from the somewhat 

 truncated base to the top of the third radials. Base depressed, or about four 

 or five times as wide as high, with slightly expanded margins notched at the 

 sutures. First radial pieces of moderate size, a little wider than long, and, 

 as usual, two heptagonal and three hexagonal. Second radial pieces a little 

 smaller than the first, more or less regularly hexagonal, the superior lateral 

 sides, however, being sometimes very short. Third radials about as large as 

 the second, normally pentagonal, but sometimes with the lateral angles trun- 

 cated so as to give them an irregular heptagonal outline. Secondary radials 

 resting one upon each superior sloping side of each third primary radial, 

 which they exceed in length ; more or less prominent, curving outward, and 

 generally constricted and rounded in the middle, though not entirely free 

 from the walls of the body, excepting on the upper side ; supporting on each 

 of their superior (free) sloping sides the first divisions of the arms. First 

 interradial pieces about as large as the first radials, hexagonal, and support- 

 ing two smaller pieces in the next range, which connect with others above, 

 belonging apparently more properly to the vault. First anal piece nearly as 

 large as the first radials, heptagonal in form, and supporting in the second 

 range two pieces, one of which (in the typical specimeu) is as large as the 

 first ; above these there are four or five smaller irregular pieces in the third 

 range, and above the latter others belonging apparently to the vault. 



Arms slender, rounded, and composed below of distinctly constricted pieces 

 longer than wide, and, after the first division on the secondary radials, the 

 inner ones bifurcating on the second piece, and one or both of these subdi- 

 visions divide again on the second piece, above which they still continue to 

 be composed of a single range of rounded pieces for a short distance, and 

 then pass gradually through a few wedge-formed pieces into a double series 

 of alternating pieces ; above this they are all a little stouter than below, and 

 show a very slight tendency to become somewhat flatter toward their upper 

 extremities. So far as can be seen, the two outer arms of each ray are simple 

 from their origin on the secondary radials, and composed of a single series 

 of rounded and constricted pieces as far up as the last bifurcations of the 

 other arms ; consequently there appear to be eight ultimate divisions, or 

 arms, to each ray. (Vault unknown.) 



Surface apparently minutely granular, and with a small, more or less de- 

 fined ridge extending from the base up the primary radials to the third radi- 

 al, on which it bifurcates and sends a branch to the base of each of the two 

 main arms. On the first radials, as well as the anals, there is also some ten- 

 dency to send off an obscure ridge across from one to another, on each side. 

 None of the body plates are tumid, though they are generally very slightly 

 convex. 



Height of body from bottom of base to the top of secondary radials, 16 ; 

 breadth, about 0-30 inch. 



This species is related to A. chloris, Hall, but is much smaller, and differs 

 in having its body, belo\f the arms, merely rather rapidly expanding, with 

 nearly straight sides, instead of being hemispherical. Its arms also bifurcate 

 differently. In the choris, for instance, they all (or at any rate, the inner 

 ones), after their origin on the secondary radials, bifurcate once on the first 

 piece beyond, while in our species the inner arms bifurcate on the second 

 piece, and the outer of those subdiv.sions again on the second piece. 



Locality a7id position. — Upper division of the Burlington member of the 

 Lower Carboniferous at Burlington, Iowa. Mr. Wachsmuth's collection. 



[July. 



