84 BULLETIN: MUSEUM OF COMPAxlATIVE ZOOLOGY. 



This has much the same form as G. acutus from the same horizon, 

 being very straight and gradually tapering, only it is about thrice the 

 size of the latter. It exhibits a length of 28 cm., a maximum width 

 of 3.5 cm. on the lateral faces, and a thickness of over 3 cm. a o is 

 too imperfect, however, for closer identification. The type of G. acutus 

 is preserved in the United States National Museum. 



Formation and Locality. — Keokuk LimCiftone ; Keokuk, Iowa. 



Ctenacanthus decussatus, sp. nov. 



Plate 6, Fig. 3. 



The specimen shown of the natural size in Plate 6, Figure 2, bears 

 the United States National Museum catalogue number 4846, and was 

 obtained from the Kinderhook limestone at an uncertain locality, but 

 presumably fi'om either Iowa or Illinois. It is preserved for a length of 

 12 cm., shows the whole of the inserted portion, and sufficient of the 



A, 



Fig. n. 



Ctenacanthus dccnssatus, sp. nov. Cross-sections of spine near point of insertion 

 and middle portion (A). X i- 



exserted to afford a fair idea of its form and surface ornamentation. 

 The spine is remarkably robust, being almost as thick as it is wide ; 

 and in this respect it contrasts strongly with other species from the 

 same horizon, the majority of which are much laterally compressed. 



In cross-section (cf. text-figure 11) the spine resembles G. buttersi 

 from the Lower Coal Measures, and G. pellensis from the St. Louis 

 limestone, but the ornamentation is different from both. The general 

 outline was probably of the same elongated nature as C. denticulatus, 

 C. depressus, and G. venustus, the last two being likewise from the 

 Kinderhook group. The sides are ornamented with prominent de- 

 cussated longitudinal costte, about 24 of which are to be counted along 



