216 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
The original of this figure belongs to the United States National Museum at 
Washington, and was collected by Mr. L. A. Cox from a quarry in the Keokuk 
limestone on Cedar Street in the city of Keokuk. The spine was nearly perfect 
when found, but was injured in extricating it from the matrix. The shaded 
portion of the summit is now to be observed only in impression on the under- 
lying slab, and the extreme tip is restored from a pencil sketch made by Mr. 
Cox according to his.recollection. The dotted lines which are intended to 
show the anterior ‘‘ shoulder ” and basal projection as they occur in most species, 
are to be understood as conjectural, and a reliable index as to the elevation 
of the summital portion is therefore wanting. The width across the anterior 
shoulder at the point where it is broken off is no less than 3.7 cm., and it is 
in this region that the greatest thickness of the walls occurs. The thickness of 
the exserted portion is indicated by the two sections that are given, the upper 
one of which shows the approximation of the pulp-cavity toward the concave 
margin. The total length of the part preserved is 32 cm., and the total height 
12cm. Some hesitation is felt in referring this specimen to S. productus, but 
this course seems preferable to recognizing it as a distinct species. 
Formation and Locality. — Burlington and Keokuk Groups; Lowa. 
Stethacanthus depressus (St. Jonny and WorrHEn). 
(Text-figure 15.) 
1875. Physonemus depressus St. John and Worthen, Pal. Illinois, Vol. VI. p. 452, 
Pix VIEL) ig.> 3: 
Only a few impertect examples of this species were known to its authors, all 
of them under two inches in length, and recognized as distinct from other 
forms chiefly on account of the 
“shoulder” being broadly 
rounded from side to side, and 
the concave margin of the ex- 
serted portion being very gently 
the spines belonging to this spe- 
cies is well shown in several 
specimens from the Kinderhook 
of Le Grand, Iowa, now in the 
collections of the United States 
3 National Museum at Washing- 
Fic. 15. ton. It is also well displayed 
iginal of t-figure 15 
Stethacanthus depressus (St. J. and W.). Wa- va Les pores oe Ss 
verly sandstone, Marshall, Mich. Right lat- W21CD Delongs presumably to the 
eral aspect of spine, X }. same species, and is from the 
Waverly sandstone of Marshall, 
Calhoun County, Michigan. Although the actual substance of this spine has 
curved. The complete outline of - 
a 
