ACANTHOSPIRA 419 



pair of slightly elevated ridges which diverge from the beak to the hinge- 

 line which they meet a little less than half-way from the margins of the 

 delthyrium to the cardinal extremities, no differentiation in the ornamen- 

 tation of the two portions of the area is recognizable in the type speci- 

 men ; delthyrium open, rather large, its width at the base about equal to 

 its height; each lateral slope marked by about seven simple, rounded, 

 radiating plications which originate along the cardinal margin, the pli- 

 cations bounding the sinus are the largest, the others growing successively 

 smaller to the last one, which is almost obsolete. 



Brachial valve depressed convex, becoming compressed towards the 

 cardinal extremities, most convex posterior to the middle, gently convex 

 to the antero-lateral and anterior margins; mesial fold non-plicate, well 

 defined, originating at the beak, rather low, flattened on top ; the beak 

 small, scarcely incurved, only slightly protuberant beyond the cardinal 

 margin; each lateral slope marked by six plications entirely similar to 

 those of the opposite valve and alternate with them. 



Aside from the plications the surface of each valve is marked by regu- 

 larly arranged, minute tubercles or papilla? which doubtless supported 

 slender spines in the living shell, a single radial row of such tubercles is 

 present upon the summit of each plication, in addition to which a single 

 row occupies the median line of the sinus of the pedicle valve, and two 

 rows the flattened portion of the fold of the brachial valve ; very fine, 

 more or less obscure, concentric lines of growth are also present upon 

 each valve. Shell structure apparently not punctate. 



Remarks. In the apparent absence of any punctate shell structure, 

 this species does not conform with the generic definition of Spiriferiria, the 

 genus to which it has been referred by its author. The internal charac- 

 ters of the pedicle valve are not clearly defined because of the complete 

 retention of the shell substance, but the valve is sufficintly transparent to 

 clearly show the presence of a pair of dental lamellae diverging from the 

 beak in line with the bounding plications of the mesial sinus, and extend- 

 ing for about one-third the length of the valve, while no median septum is 

 apparent and is probably wanting in the species. The absence of the 

 punctate shell structure, the dental lamellae not united to form a spondy- 

 lium, and the open delthyrium likewise exclude the species from Cyrtiua. 

 Syringothyris and its allies are also excluded by reason of the non-punctate 

 shell and the absence of the peculiar surface markings of those genera, 

 and no delthyrial plate or syrinx is known to be present. The species 

 agrees most closely with the members of the genus Spirifer, but the pres- 

 ence of the regularly arranged, minute spines of a character foreign to that 

 genus. The species is, therefore, considered as the type of the new genus 

 Acanthospira. 



Horizon. Louisiana limestone. 



