84 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
This has much the same form as C. 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. It is 
too imperfect, however, for closer identification. The type of C. acutus 
is preserved in the United States National Museum. 
Formation and Locality. — Keokuk Limestone ; Keokuk, Iowa. 
Ctenacanthus decussatus, sp. nov. 
Plate 6, Fig. 2. 
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 from 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. 
Cee OE 
Ctenacanthus decussatus, sp. nov. Cross-sections of spine near point of insertion 
and middle portion (A). X 4. 
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 C. buttersz 
from the Lower Coal Measures, and C. 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 C. venustus, the last two being likewise from the 
Kinderhook group. The sides are ornamented with prominent de- 
cussated longitudinal costw, about 24 of which are to be counted along 
a 
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