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INFERIOR OOLITE AMMONITES. 



This species may be said to be fairly uniform, as it varies but little, except in 

 the direct genetic line. It is the descendant of Sonninia irregularis — to be very 

 precise, it comes from a form exactly between irregularis a and j3. I could 

 figure every gradation between the two species, each gradation showing the 

 gradually earlier and earlier change from the spinous to the costate stage. In 

 the figured specimen of irregularis the last spine is produced when the shell is 

 nine inches (228 mm.) in diameter ; in the figured example of this species the last 

 spine appears at a diameter of about 5-§- inches (54 mm.). 



Thus the costate stage is the prominent feature of the fossil, a fact which 

 effectually separates it from any of the species previously figured. The nearest 

 morphological equivalent is Bonn, acanthodes a ; but the earlier disappearance of 

 the spines, the flatter, more compressed whorls, and the nearly upright ribbing of 

 marginata, show that the present species is a much further retrogression in a 

 somewhat different direction. In the aged specimen (PI. LXIV) it should be 

 noticed in reference to future developments that the ribs become proportionately 

 larger and more distant. Something of the same feature may be seen in the 

 specimen shown in PI. LXII. The senile stage of the aged specimen should be 

 the morphological prefiguration of the adult of the next species. 



Sonn. marginata owes its name to its well-marked inner edge, though it shares 

 this feature with other species. It occurs in the Goncavum-zone of Bradford 

 Abbas, Halfway House, &c. 



A typical specimen of the species is figured in PI. LXII, showing that the 

 irregularly-spinous stage of irregularis has become a youthful feature. A front 

 view of this fossil is given in PI. LXIII, fig. 2. In PI. LXIV a grand aged 

 example of the species is figured, reduced two-thirds of natural size, illustrating 

 the acquirement of coarser, more distant costge in old age. Fig. 2 gives an outline 

 of the aperture similarly reduced ; while in fig. 3 the last suture-line is depicted 

 of natural size. The branching of the superior lateral lobe is somewhat unusual 

 and irregular. 



In PI. LXV are figured two views of an immature specimen of this species in 

 order to compare with the figured examples of crassispinata and acanthodes of the 

 same age. The side view is given in fig. 1 ; the front view in fig. 2. 



Sonninia dominans, 8. Buchnan. Plates LXVI ; LXVII, figs. 1 and 2 ; Plate 



LXIX ; and, an intermediate form, Plate 

 LXVII, figs. 3-5. 



Discoidal, compressed, hollo w-carinate. Whorls, in section, sub-quadrangular, 

 oblong, ornamented with direct, upright, ventrally-inclined ribs, which in the 



