90 TRANS. OF THE ACAD. OF SCIENCE. 



beneath the foramen, cutting its periphery. Spiral appen- 

 dages large, with about 11 volutions on each side. Surface 

 ornamented with numerous concentric, pectinated lamellae; 

 the teeth or spines often lie in longitudinal fascicles on the 

 border, giving the appearance of plications. The cast is 

 marked with indistinct longitudinal plications or costse. 



Length of medium specimen, .07 ; width, .68 ; thickness, .40. 



Length of small specimen, .39; width, .37; thickness, 22. 



This beautiful fossil most resembles the Sp. pianos ulcata, 

 of Prof. Phillips; but its beak is larger and less incurved; its 

 dorsal valve more convex, and its ventral one less so. The 

 Spirigera hirsuta of Prof. Hall has a larger beak, a more 

 convex ventral valve, and a more flattened dorsal one, and 

 only about half as many volutions in the spiral appendage. 



This Spirigera is very abundant in the 2d Archimedes or 

 Ste. Genenvieve limestone at Chester, Illinois, at St. Mary's, 

 and other places in Missouri. It is associated with Spirifer 

 JLeidyi, S. sjnnosus, and JProductus elegans. 



Spirigera caput-serpentis. 



Shell large, gibbous or depressed, wide, ovate, trilobate, 

 costate and striate. Ventral valve very convex near the beak 

 and on the raised lobes on each side of the sinus; strongly 

 arched longitudinally: sinus deep and wide in front, obso- 

 lete towards the beak ; angular at the bottom : umbo full : 

 beak strongly incurved, truncate: foramen circular or ovate; 

 plane, parallel to the junction of the anterior portion of the 

 valves. Dorsal valve suborbicular or subquadrate ; flattened 

 towards the lateral margins; nearly straight from the umbo 

 to the anterior margin : mesial fold full, broad, round, having 

 a short, deep, round sinus on eaeh side at the anterior margin, 

 making the junction of the valves very sinuous: umbo full; 

 beak small, strongly incurved beneath, and cutting the peri- 

 phery of the foramen. Spiral appendages large; at least 

 twenty volutions on each side. Surface marked with distinct 

 lines of growth, by fine concentric striae, and by obsolete, de- 

 pressed, longitudinal costse, which become more obvious in 

 exfoliated or worn shells — they are still more distinct on the 

 cast. Young shells less convex, with a thin linguiibrm, up- 

 turned projection in front. 



Adult shell— length, 1.54; width, 1.56; thickness, 1.08. 



Young shell — length, 1.03; width, 1.05; thickness, .66. 



This fossil most resembles the Spirigera subtilita of Prof. 

 Hall, but it is larger, wider in proportion, and more trilobate. 

 It may be distinguished from the S. differentiae of Prof. 

 McChesney by its ovate and trilobate form; it is always 

 wider below the middle. The S. Charitonensis is orbicular, 



