GRAMMOCERAS LEURUM. 195 



Grammoceras LEURUM, 8. BucJcman. Plate XXXIII, figs. 5 — 10. 



1885. Ammonites cf. Ltthensis, Quenstedt. Amm. Schwabischen Jura, pi. liv, 



figs. 54, 55. 



Discoidal, compressed, carinate. Whorls broad, very slightly convex, 

 ornamented in youth with subsigmoidal radii, in adult almost perfectly smooth. 

 Radii in the inner ai'ea somewhat distant, branching into twos or threes on the 

 outer area, and also on this area are occasional intermediate ribs ; at intervals a 

 rib larger than its fellows crosses the side ; the radii becoming finer, die away first 

 on the outer area, and finally disappear altogether. Ventral area scarcely defined, 

 the two sides falling towards a small but distinct solid carina. Inner margin 

 upright and flat. Inclusion about one-half. Umbilicus small, marked with some- 

 what coarse ribs. Aperture acutely sagittate. Suture-line simple, superior lateral 

 lobe really no longer than siphonal lobe. 



Like Gramm. suhserrodens this species has, in general, all the appearance of a 

 Lioceras ; but the simple suture-line shows that it does not belong to that genus, 

 and that it is only an involute development of Orammoceras. I believe that the 

 parent of this species is either Gramm. aalense or Gramm. coshdatum (p. 197) ; and 

 I incline to the former, because the ribs in some specimens appear to be more 

 numerous than I should expect in the descendant of costulatum. Still, it is a point 

 which cannot be absolutely decided without more material. The transition from 

 aalense (PI. XXXII, fig. 7), or from costulatum, to this species is but a step, which 

 may be expressed as a further development of involution,^ accompanied by an 

 earlier acquirement of the senile — the smooth — stage. 



The above remarks will show how this species differs from Gramm. aalense or 

 from Gramm. costulatum, to the former of which it stands in the same relation 

 as the species to which it converges, namely, Gramm. suhserrodens, does to 

 Gramm. mactra. 



This convergence to Gramm. suhserrodens is a curious fact ; but I believe that 

 it is the true explanation of the case, and not that this species is a coarse-ribbed 

 variety of that one. Practically speaking, it differs from Gramm. suhserrodens 

 only in the coarseness of its ribbing, and the absence — but this is rather remark- 

 able and striking — of those fine, hair-like striae ; because almost as soon as the 

 ribs die away the specimen appears perfectly smooth. It is on account of this 

 smooth character that I have made use of the name leurum i\iv^oQ=z smooth). 



There is a noticeable, but possibly not important, difference in the sutures of 



the two specimens figured. Fig. 10, which belongs to the specimen shown in 



^ To see the difference between Gramm. costulatum and this species in the size of umbilicus requires 

 older specimens of the former, of which the umbilicus is one-fourth larger. 



