16 THE ■EYOl.VTIO^ OF INOCERAMUS, [Feb. 1912. 



I.inconstansWoods (%s. 65-80 & 86,pp. 14-15 & 17)* extends from 

 the zone of Holaster lolanus to the zone of Belemnitella mucronata, 

 and perhaps occurs also in the zone of Terehratulina lata. It shows 

 a large amount of variation. The less convex forms of this species 

 approach very closely /. labiatus var. latus ; but the hinge usually 

 is relatively longer, the height of the skell less, the axis of growth 

 more oblique to the hinge, the flattened area larger, the umbones 

 less prominent, and the posterior ear more distinct. 2. inconstant 

 has undoubtedly been derived from the labiatus stock, and came 

 probably from /. labiatus var. latus, but possibly from the longer 

 and less high form of /. labiatus which occurs in the zone of 

 Rhynchonella cuvieri. 



From the variety of /. ineonstans which throughout life is nearly 

 flat or but slightly convex (fig. 67, p. 14), numerous varieties have 

 arisen in which, sooner or later, the shell becomes convex or inflated 

 owing to the later part growing towards the plane of the valves 

 instead of nearly parallel to it : consequently the later- formed part 

 curves more or less rapidly from the earlier part. The flat stage 

 may last for a long (fig. 67) or a short period (figs. 71, 74), and in 

 one variety (sarumensis, found in the Actinocamax-quadratiis Zone, 

 figs. 82 & 83, p. 15) disappears altogether, with the result that a 

 regularly convex shell, without folds, is formed. Sometimes the 

 change from the flat to the convex stage is abrupt, so that a sharp 

 bend in the shell occurs (figs. 71-73, p. 14) ; in other cases, the 

 change is gradual, and a dorso-ventral section of the shell is convex 

 (figs. 74 <fe 75). When the flat stage is of short duration, the an- 

 terior flattened area becomes relatively shorter, and in the variety 

 sarumensis (figs. 82 & 83, p. 15), in which the flat stage is lost, the 

 anterior area is absent, and the umbo is not terminal. 



/. ineonstans var. striatus Mant. (figs. 84 & 85, p. 15),^ found in 

 the zones of Holaster planus and Micraster cor-anguinum, is another 

 form in which the flat stage is absent ; the valves are inflated, the 

 folds indistinct, and the anterior area absent or indistinct. 



/. balticus Bohm (figs. 87-89, p. 17) ^ ranges from the zone of 

 Marsupites testudinarius to the zone of Belemnitella mucronata. In 

 some examples of the slightly convex form of /. ineonstans the hinge 

 is relatively longer than usual (fig. 86), and these approach very 

 closely the shorter and less convex forms of /. balticus (fig. 87) and 

 also resemble the early stage of the large, convex form.* Other 

 examples of /. balticus (especially older individuals) are much 

 longer, and the axis of growth becomes more oblique to the hinge- 

 line ; the valves become much more convex, owing to the change 

 in the mode of growth like that already described in /. ineonstans. 

 It seems clear that I. balticus has been derived from /. ineonstans 

 by an increase in the relative length of the shell. 



1 H. Woods, op. cit. pi. li, figs. 1-4 & text-figs. 39, 42-49. 



2 Id. ibid. pi. li, fig. 5 & pi. lii, fig. 1. 

 ■" Id. ibid, text-figs. 51-53. 



^ Id. ibid, text-fig. 53. 



