542 > GEOLOGY OF OHIO. 



diameter, the siphuncle was of the same width. In a second specimen 55 

 mm. long, with 20 septa, the centre of the siphuncle was moderately excen- 

 tric at the smaller end with a diameter of 15.5 mm., the siphuncle being 2.6 

 mm. wide. Where the shell has a diameter of 18 mm. the siphuncle 

 is 2.8 mm. wide, showing a slight accidental increase of its diameter; 

 here the centre of the siphuncle lies only 8 mm. from the side of the 

 shell. From this point the diameter of the siphuncle decreases so 

 rapidly that the fifth succeeding segment is only 2.2 mm. wide. At the 

 .larger end where the shell has a diameter of 20.5 mm., the siphuncle is 

 2.1 mm. wide, and while approaching very closely to a cylindrical 

 siphuncle, it yet shows a moderately convex outline in longitudinal sec- 

 tions. Another specimen with 17 septa, 47 mm. long, shows an annu- 

 lar siphuncle 2.8 mm. wide at the smaller end, which has a diameter of 

 lo mm., and an almost cylindrical siphuncle only 1.8 mm. wide at the 

 larger end where the 'shell has a diameter of 20.5 mm. The septa de- 

 scribe an arc. of 80°. Figs. 1, 2, 3, on page 540). 



Orthoceras {Eu- Orthoceras) . 



('Fig. 4 on page 540). 



Specimens of a small Orthoceras occur having septa at distances 

 intermediate between those of O. erraticum. and O. Hanover ense, but 

 the shells show neither the markedly variable siphuncle of the first nor 

 the peculiar groovings of the internal cast of the last species. Good 

 specimens of Orthoceras are not common enough in the Ohio Clinton to 

 settle all doubtful points and in the case of the specimens just mentioned 

 it will be necessary to let their position remain doubtful for the present. 



At Todd's Fork was found a specimen tapering in a length of 44 

 mm. from 8 mm. to 14 mm. It had a smooth shell ; there were 26 

 septa, which varied considerably in convexity. The siphuncle was ann- 

 ular during its entire length and, where the shell had a diameter of 12 

 mm., was 1.8 mm. wide. The siphuncle changed its position slightly 

 during its growth, but the change of position was too small to have any 

 special significance. (Fig. 4 on page 540). 



Several small fragments of the same type were found at Hanover, 

 Indiana, though they were too small to show anything but the annular 

 siphuncle and too short to show any considerable change of position of 

 the siphuncle, even if any occurred. 



A specimen from Brown's Quarry with smooth shell, and septa 

 approaching each other as closely as those of the forms just described, 

 is doubtfully referred here. The siphuncle is not preserved. 



Specimens in the form of casts with septa at distances from each 

 other corresponding to those in O. erraticum, and with slightly eccentric 

 siphuncle, are common at Todd's Fork, but the form of the segments of 



