AMERICAN PALEOZOIC CEPHALOPODS 
229 
mid-height. This transverse line indicates where the calcareous 
deposits enveloping the successive septal necks of the siphuncle 
meet, as in other Actinoceroids. At the top of the specimen, the 
interior of the siphuncle shows an irregular central depression, 
probably corresponding to the so-called endosiphuncle of Acti- 
noceroids. 
The interior of the camerae is occupied by a calcareous de- 
posit, showing a few cystoid markings, and also a tendency to- 
ward pseudo-septa. 
The exterior surface of the shell is striated transversely by 
narrow bands, abruptly limited along their upper margins, but 
more gradually sloping toward their lower margins. About 14 
to 16 transverse striae occur within a length of 10 mm. On the 
dorsal and lateral sides of the conch these striae are directly 
transverse, but along the ventral side they curve gently down- 
ward. 
Locality and Horizon. — St. Paul, Indiana, in the Laurel lime- 
stone (Silurian). 
Specimen No. 633, in the Indiana State Museum, at Indian- 
apolis, Indiana. 
Hartsville type . — The second specimen figured by Miller, in 
his original description of Cyrtoceras indianense, consists of 11 
camerae. The lengthwise convex curvature of the ventral side 
along the 3 lower camerae has a radius of 45 mm. ; the fourth 
camera is distinctly less curved, and the ventral side of the up- 
per part of the conch is straight. In a corresponding manner, the 
dorsal side of the specimen is concavely curved along the 3 or 4 
lower camerae, and becomes straight farther up. The dorso- 
ventral diameter enlarges from 29 mm., at the top of the third 
camera from the base, to 36 mm. at the top of the seventh camera, 
the intervening distance being 45 mm. This suggests an apical 
angle of 9° along the straight part of the specimen. The speci- 
men is curved laterally, especially along the lower part. 
There are three camerae in a length equal to the dorso-ventral 
diameter of the conch at mid-length of the specimen. The upper 
four segments of the siphuncle are exposed. Their height is 11 
mm. They are represented by deposits lining the interior of 
their walls. From these deposits it is evident that the diameter of 
the segments enlarges scarcely 2 mm. on passing from their con- 
tact with the septa to their greatest diameter at mid-height. 
Where the dorso-ventral diameter of the conch is 36 mm., the 
