ORTHOCERAS. 151 



fracture. The rate of tapering seems greater in this species along the major 

 than along the minor diameter. This specimen also retains an interesting segment 

 of the siphuncle, free from matrix, and showing its beaded shape. 



Another of Mr. Vicary's specimens, which has lost its shell, has a low rounded 

 carina running longitudinally across the segments. A similar carina is seen 

 in the figures of 0. serratulum, Barrande,^ and of 0. Marcellense, Hall.^ 



A third fossil in the same collection I have referred with some doubt to this 

 species. It is a cast of a much smaller shell. In it the septa are decidedly closer, 

 but their ratio to the diameter remains about the same. They are also very 

 oblique to the axis of the shell, as will be seen by reference to its figure (Plate XV, 

 fig. 7) ; but this appearance is perhaps due to the particular side which is exposed, 

 and which cannot be so well seen in Phillips's typical specimen (fig. 8). Its rate 

 of tapering seems to be about the same. It appears, therefore, that, though 

 there are no data at present for removing it from the present species, there is 

 a possibility that a more perfect example, if found, might prove it to belong to 

 some other form. 



Affinities. — In 0. lineare, Miinster,^ a fine microscopical striation is present ; 

 0. regulare, Schlotheim, has, according to Miinster,* more distant septa, which 

 are almost as much as their width apart; while 0. venustum, Miinster/ also 

 differs in the great excentricity of its siphuncle. 



P. A. Romer" appears to identify 0. regulare, Schlotheim, with a shell 

 different from the one which Mimster has referred to it ; and possibly he may 

 have allocated more than one species under that name. He states his shell 

 to be smooth, but the septa are narrower than those of Phillips's 0. cinctum, 

 and it approaches 0. laterale, Phillips, more nearly than the present form. 

 0. virgatum, Sowerby,'^ as given by F. A. Romer,^ seems very similar, but differs 

 in having more distant septa, and in being ornamented with longitudinal striae. 

 A reference to Sowerby's original species shows that whether it be the same as 

 Romer's version of it or not, it is quite distinct from the shell under consideration, 

 and that it has these striae very large and coarse. 0. subconicum, F. A. Romer,^ 

 tapers much more rapidly, and has more distant septa. 0. incequale, F. A. 



1 1868, Barrande, ' Syst. Sil. Bobeme,' vol. ii, pt. 3, p. 550, pi. ccxcviii, figs. 6—13, Et. E. 



2 1879, Hall, ' Pal. N. Y.,' vol. v, pt. 2, p. 278, pi. xxxviii, figs. 4—9 ; pi. Ixxxiii, figs. 1—12; and 

 pi. cxiii, fig. 18. 



3 1810, Miinster, ' Beitr.,' pt. 3, p. 99, pi. xix, fig. 1. 



4 Ibid., p. 95, pi. xix, figs. 3, 4. 



5 Ibid., p. 98, pi. xviii, fig. 6. 



6 1843, P. A. Komer, ' Verst. Harz.,' p. 35, pi. x, figs. 4, 5, 8, 9. 



7 1839, Sow. in Murch., 'Sil. Syst.,' p. 620, pi. ix, fig. 4. 



8 1843, P. A. Eomer, ' Verst. Harz.,' p. 36, pi. xii, fig. 37. 



9 1850, P. A. Eomer, 'Beitr.,' pt. 1, p. 17, pi. iii, fig. 20. 



20 



