214 BULLETIN: MUSEUM OF COMPAKATIVE ZOOLOGY. 



advanced stage of modification than this is exemplified by the various species 

 of Stethacanthus, the basal portion having become much elongated and the 

 anterior "shoulder" very conspicuous. In the more primitive forma the 

 summit projects but slightly above the line of insertion, and the concave mar- 

 gin is very gently curved. Gradually the summit becomes more strongly and 

 abruptly elevated, finally assuming an erect or even recurved position, as in 

 S. altonensis and soAe other species. 



The anterior tumid portion or " shoulder " which terminates the exserted 

 portion in the line of the body-wall in front, doubtless served as a buttress for 

 strengthening the attachment of the spine in the soft parts and forms the ini- 

 tial portion of the cutwater. The depth to which the spine was inserted, or, in 

 other words, the line of the body-wall, is denoted by the regular termination of 

 the coarse vascular impressions which are limited to the basal portion. The 

 latter is always more or less produced in advance of the anterior shoulder, and 

 in some species it is also produced posteriorly beyond the point where the ex- 

 serted portion enters the integument, as shown in Newberry's figure of S. alto- 

 nensis.'^ This author's observation that "the base shows the outline of what 

 seems to be a spheroidal head that fitted into the socket of an articulation " 

 may be dismissed as having no foundation of fact, and probably arose from 

 deceptive appearances. 



The Devonian spines known as Acantholepis and Phlyctaenacanthus also ex- 

 hibit a very large internal cavity, and appear to have been inserted in an 

 essentially similar manner. 



Stethacanthus altonensis (St. John and Worthen). 



1875. Physonemus altonensis St. John and Worthen, Pal. Illinois, Vol. VI., p. 454, 



PI. XIX., Figs. 1-3. 

 1875. Drepanacanthus reversus (errore) St. John and Worthen, Ibid., p. 457, Pi. 



XIX., Fig. 6 a. 

 1889. Stethacanthus altonensis J. S. Newberry, Monogr. U. S. Geol. Surv., Vol. 



XVI., p. 198, Pi. XXIV. 



This, the typical species, appears to be restricted to the 8t. Louis limestone, 

 and in its larger size and nearly erect sunmiit represents a more advanced stage 

 of modification than the Burlington species. A specimen larger than any de- 

 scribed belongs to the private collection of Dr. G. Hambach, in St. Louis, and 

 has a total length of 24 cm., the basal portion being conspicuously produced 

 beyond the limits of the exserted part both in front and behind. The walls 

 have a uniform thickness of about 2 mm. throughout, except along the cut- 

 water and posterior margin of the exserted portion. Newberry's statement 

 that these spines exhibit a want of bilateral symmetry is clearly erroneous. 



Formation and Locality. — St. Louis limestone ; Illinois and Missouri. 



1 Monogr. U. S. Geol. Surv., Vol. XVL, 1889, PI. XXIV. 



