226 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
Fig. 51. Platysomus circularis Newb. and Worth. Coal Measures; Mazon Creek, 
Grundy County, Illinois. Complete fish with well-preserved fins and 
squamation, X 3. 
Fig. 52. Chedrodus orbicularis (Newb. and Worth.). Coal Measures; Mazon Creek, 
Grundy County, Illinois. Complete fish with well-preserved fins and 
dorsal and ventral peaks, X 3. 
Fig. 58. Deltodus occidentalis Newb. and Worth. (var. D. latior Newb. and Worth.). 
Keokuk limestone; Keokuk, Iowa. Posterior dental plate referred to 
right ramus of the lower jaw. 
Figs. 54, 54a, 54. Helodus incisus, sp.nov. Mississippian; Salem, Indiana. Sup- 
posed symphysial tooth corresponding to the form known as “ fTelodus 
coxanus’’ Newb. Seen in anterior, posterior, and lateral aspects, 
ee 
Fig. 55. Deltodus spatulatus Newb. and Worth. Burlington limestone ; Burling- 
ton, Iowa. Anterior dental plate referred to the left ramus of the 
lower jaw, and described by Messrs. St. John and Worthen as a 
distinct species (“ D. convolutus’’), X 4. 
Fig. 56, 56 a. Tuberculated dermal plate of an undetermined Elasmobranch. 
Kinderhook limestone ; Burlington, Iowa. The unsymmetrical cross- 
section near the base is shown in Fig. 56a, X 4. 
Fig. 57. A spiniform dermal tubercle of the same nature as that shown in Fig. 
56, the external surface much corroded and displaying the fibrous 
internal structure, X +. 
Fig. 58. Homacanthus acinaciformis, sp. nov. Chemung Group; Warren, Penn- 
sylvania. Lateral aspect of spine lacking inserted portion, X +. 
Fig. 59. Homacanthus delicatulus, sp. nov. Kinderhook limestone; Le Grand, 
Iowa. The same spine as shown in Plate 3, Fig. 28, four times 
enlarged. 
The originals of Figures 48, 51, and 52 are preserved in the Peabody Museum 
of Yale University ; the remainder in the Museum of Comparative Zoology. 
