FOSSILS OP THE CLINTON GROUP. 541 



Orthoceras described on page 542 Fig. 4, Todd's Fork, Ohio. 



Orthoceras ignotum, Foerste, Fig. 5, a medium sized specimen ; section show- 

 ing siphuncle, Soldiers' Home, Ohio. Fig. 6, interior cast, Huffman's Quarry, Ohio. 

 Fig. 7, section of fragment from large sized specimen, Soldiers' Home, Ohio. 



Orthoceras (Eu- Orthoceras) erratic'um, Foerste. 

 (With characters of Aclinoceras when young). 



(Figures 1, 2, S. on page 540.) 



In the "Orthoceras block" at Huffman's Quarry were found rather 

 abundant fragments of a form which, shows about the same rate of taper- 

 ing as O. ignotum. In several cases, the more aged parts of the shell, 

 where they Were 24 mm. wide, practically ceased tapering, producing in 

 this manner a cylindrical growth in the later formed parts of the shell. 

 These shells were very instructive in showing certain variations which 

 siphuncles may undergo not only in the same species but even in the same 

 shell. The remarkable fact was discovered that fragments more than 18 

 mm. in diameter showed cylindrical siphuncles, and those less than 16 

 mm. in diameter had siphuncles whose segments were more or less 

 strongly contracted towards the septa, thus producing the appearance of 

 a close string of beads. Moreover in three cases the gradual change of 

 the siphuncle from an annular or bead like one to a cylindrical one 

 could be distinctly traced. Judging from this series of specimens it 

 would therefore seem that the annular siphuncle was the earlier form 

 from which the cylindrical form was a later development. Moreover the 

 position of the siphuncle varied in quite a number of specimens in such 

 a way that the siphuncle, which was annulated and excentric at smaller 

 diameters of the shell, became central while becoming cylindrical, and 

 then continued to change its position in the same direction so that the 

 more recent cylindrical part of the siphuncle was again excentric, but 

 situated on the opposite side of the shell from the siphuncle in the earlier 

 part of its growth. The freedom with* which the siphuncle has changed 

 its place in these specimens is remarkable, and may well nigh be regarded 

 as a characteristic of the species. Whereas it has been observed before 

 that the position of the siphuncle was variable in the same species, its 

 position in the same individual has usually been found to be more con- 

 stant. Moreover in the other Clinton species so far examined the varia- 

 bility of the siphuncle has been slight. It will be instructive, therefore, 

 to add a few measurements showing this variation in the Orthoceras 

 block specimens. In one specimen 48 mm. long, with 18 septa, the 

 centre of the siphuncle was 4 mm. away from the shell at the smaller 

 end, and had passed to the centre near the larger end. For this entire 

 length the segments of the siphuncle were very distinctly and almost 

 equally contracted towards the septa; -at the smaller end, 11 mm. in 

 diameter, the siphuncle was 2.8 mm. wide; at the larger end 16 mm. in 



