540 



GEOLOGY OF OHIO. 



Fig. 5 on this page.) A specimen from Soldiers' Home retains 

 the diameter of 12.5 mm. in a length of 21 mm. including 8 septa; 

 the cylindrical siphuncle is 1.7 mm. wide; the surface of the thin shell is 

 smooth. The very low rate of tapering shown by this specimen is 

 worthy of note, but is not sufficient for its separation from the previous 

 ly described forms. A more typical but much smaller specimen was 

 found at Todd's Fork, in the upper ferruginous layer. It was 35.2 mm. 

 long, with diameters at its extremities of 10.5 mm. and 8.2 mm.; at the 

 larger end the cylindrical siphuncle was 1.4 mm. broad, the specimen 

 showed 15 septa; the shell was thin and smooth, the arc described by 

 the septa was only 80°, but this does not warrant the separation of this 

 specimen from the species. 



A single specimen from the Soldiers' Home presents many char- 

 acteristics of O. ignotum; the shell is rather thin and seems to have been 

 smooth, the septa describe an arc of 105°, the siphuncle is 2.9 mm. 

 wide, and is cylindrical; the shell has a diameter of 22 mm. In two im- 

 portant particulars this specimen however differs from the former. In 

 a length of 37 mm., it maintains the same diameter, and in this 

 same length it has only 7 septa, whereas the more typical forms of equal 

 size would show 9.5 septa. Fig. 7 on this page — a partial figure. 



Specimens showing the same rate of expansion and the same relative 

 distances between the septa as Orthoceras ignotum are known from 

 Todd's Fork. Their shell is quite thick, A to .5 mm. Their siphun- 

 cle is unknown but they very probably belong to this species. Several 

 specimens are in the cabinet of Dr. Chas. Welch. 



Orthoceras (Eit-Or/hoceras) erraticum, Foerste, Figs. 1, 2, 3, showing the 

 change from an annular to a more cylindrical siphuncle. Figs. 2 and 3 show the 

 variation in position of the siphuncle. Fig. 1 is cut transversely to this plane of 

 variation and so does not show the change in position of the siphuncle. " Orthoceras 

 block," Huffman's CHiarrv, Ohio. 



