538 ' ; GEOLOGY OF OHIO. 



i 



wide at these extreme points. The septa near the smaller end showed arcs 

 of 90°. This is considerably less than in the case of the type specimen, 

 yet, all other features being identical, the .variation in the degree of con- 

 cavity of the septa only serves to indicate to what extent this feature is 

 variable. The exterior surface is smooth. The shell itself was thick. 

 Near the larger end of the Ohio specimens just described the shell- had 

 a thickness of 8 mm. 



Orthoceras (Actinoceras) clavatum, Hall. 



(Plate 33, Fig. 2; Plate 36, Figs. 5, a. b c.i 



i The type of O. rhythmoides was found at Brown's Quarry (Proc. 

 Boston Soc. Nat.Hist., 1889.) It showed rather closely placed septa, and, as 

 far as we could judge, the rate of increase in diameter of the shell was 

 very small, A section across the same showed no siphuncle. A second 

 specimen in the collections of the Ohio State University having septa 

 disposed at similar intervals, and' describing similar arcs, but possessing 

 a greater rate of increase in diameter undoubtedly belongs to this species 

 and sheds much light upon the same. It is 56 mm. long, and presents 

 in this distance, 21 septa. The diameters at the opposite ends are 27 

 mm. and 19 mm. respectively. The siphuncle is contracted at the 

 septa producing bead like segments; at opposite ends of the specimen 

 their width is 4 mm. and 3 mm. respectively. A tiny remnant of the 

 surface seems to indicate that the shell was smooth. This second speci- 

 men resembles closely the cast figured by Hall from the Clinton of New 

 York as 0. clavatum, with which the Ohio species is therefore provision- 

 ally identified. I 



Orthoceras {Actinoceras) lata-nummulatum, Foerste. 



(Plate 33, Fig. 4.) 



The type came from Soldiers' Home (Proc. Boston Soc. Nat. Hist., 

 1889.) The large size of the species and its very broad strongly annulated 

 siphuncle are its chief distinctions. 



> A fragment of some very large and closely related species is represented 

 by a fragment in the collection of Geo. Caswell. Its siphuncle is very 

 large and is strongly excetitric. Plate 35, Figs. 4, a, b, c. ■ 



* Orthoceras {Actinoceras) turgido-nummulatum, Foerste. 



(Plate 33, Fig. 7 ; Plate :'5, Figs. 1, 2.) 



The type specimen from Soldiers' Home shows an annulated siphun- 

 cle which is rather broad, considering the diameter of the shell. (Proc. 

 Boston Soc. Nat. Hist.,, 1889.) This seems to be its chief distinction from 



