31 



tween a paii" of primaries. When the latter is the case, one of the inter- 

 vening ribs is unusuall}' long- and almost joins one of the primaries, so 

 tliat a tendency to bifurcation is then observable. The whole of the ribs 

 areattirstcrowdeil and tine (except near the sutures) but at a little distance 

 from the aperture they get much wider a])art. Thej^ are prominent, 

 regular and acute (though sometimes, under the lens, they appear a little 

 rounded at their summits), and the grooves between them are rather 

 deep]}' concave. In the earlier part of the outer whorl, the grooves are 

 a little wider than the ribs which bound them, they (the grooves) gra- 

 dually increase in width, until, near the a})erture, they are about thi'ee 

 times as wide and much shallower in proportion than at the commence- 

 ment of the volution. The I'ibs, too, are more acute and prominent near 

 the outer termination of the shell. 



Septation unknown. 



Where the specimen is least distorted the greato t diameter is about 

 two incites, and the maximum width of the whorl (or thickness of the 

 shell) is nearly one inch and a half. 



It is doubtful whether the four Ammonites described above should be 

 regarded as ditlci-cnt stages of growth ot one shell, or as two, or even 

 three, distinct species. Form A. and form B. are each represented by 

 a single specimen of about the same size. Notwithstanding its globose 

 shape, and the much greater involution of its whorls, it is easy to under- 

 stand that Form B. may be the young of the type of A. Loganianus, as 

 there is little essential difference m the style of costation of both. The 

 sculpture of Form A. is certainly coarser and its ribs are much more 

 distant than is the case in any of the other three specimens ; the costs? also 

 aj^pear to Ix- moi-e obtuse and angular, but as the surface is much water- 

 worn, it is not safe to attach any importance to the latter character. In 

 Form A. one volution and a half are visible externally, and in Form B. 

 only one can be seen, but this difference may have resulted from the 

 peculiar distortion to which the last named specimen has been subjected. 

 On the whole, it is most probable that these four Ammonites belong- 

 to one species, of which Form A. may constitute a well marked variety. 



In many respects, .iwimontYf 6 Loganianus is nearly allied to the A. Gervillei 

 of Sowerb}'. Form B., in particular, can scarcely be distinguished from 

 the shell figured by D'Orbigny,* as the young state of A. Gervillei. In 

 more full}' grown specimens, the diffei-ences between the two species are 

 obvious; .1. Gervillei \s then much the most globose shell of the two, and 



* " Paleontologie FraiHiiise. Terrains Jurassiques." Vol II, Atlas, Plate CXL 



