216 



BULLBTIN : 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 tlie point where it is broken off is no less than 3.7 cm., aud 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 

 12 cm. Some hesitation is felt in referring this specimen to S. prodiictus, but 

 this course seems preferable to recognizing it as a distinct species. 



Formation and Locality. — Burlington and Keokuk Groups ; Iowa. 



1875. 



Stethacanthus depressus (St. John and Worthen). 



(Text-figure 15.) 



Physonemus depressus St. Jolm and Worthen, Pal. Illinois, Vol. VI., p. 

 Pi. XVIII., Fig. 3. 



452, 



Only a few imperfect 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 

 curved. ' The complete outline of 

 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 

 National Museum at Washing- 

 ton. It is also well di.splayed 

 in the original of Text- figure 15, 

 which belongs presumably to the 

 same species, and is from the 

 Waverly sandstone of Marshall, 

 Calhoun County, Michigan. Although the actual substance of this spine has 



Fig. 15. 



Stethacanthus depressus (St. J. and W.). 



verly sandstone, Marshall, Mich. 

 eral aspect of spine, X \- 



Wa- 

 Right lat- 



