352 INFERIOR OOLITE AMMONITES. 
This is not an uncommon fossil of the Concavwm-zone; and it occurs at 
Bradford Abbas. The side view (portion) and the outline section of the whorl of 
the thick form are shown in figs. 1, 2 of Pl. LXXXI; the side view and outline 
section of the whorl of a more costate and thinner form are illustrated in figs. 
3,4. The specimens depicted are immature. 
Sonninta Nova, 8S. Buckman. Plate LXXXII, figs. 3, 4. 
Discoidal, compressed, carinate. Whorls, in section, oblong, ventrally-conver- 
gent, practically without ornament. Ventral area arched, divided by a small, 
presumably solid carina. Inner margin well-marked, broad, upright, flat, upper 
edge rounded off. Inclusion about one-third—more in the young. Umbilicus 
excentric, regularly graduated, shelving outwards, ornamented with very rudi- 
mentary and obscure coste. 
The connection of this species with crassimuda a is effected through a 
specimen exactly intermediate in every respect. That specimen shows in its 
inner whorls a rudimentary spinous stage followed by a distinct costate stage ; 
but the greater part of the fossil is smooth. In muda there appears to be the 
least trace of a spinous stage followed by a very rudimentary costate stage, and 
then smoothness. 
In mode of coiling the intermediate form bears out its position exactly, for it 
is more umbilicate than nuda, with less tendency to excentricity ; on the other 
hand, its umbilicus is more excentric than that of crassinuda. Further, in regard 
to thickening of the whorl it is also intermediate. | 
The similarity between nuda and simplex is obvious. Being able to follow out 
nearly every step in the development of the two genetic series, I claim that the 
correct interpretation of the likeness is that these forms are the terminal 
morphological equivalents of two homoplastic series. 
The two forms have not attained to anything lke identity,—a much more 
perfect convergence is theoretically possible. From simplex the present species 
may be known by possessing a carina for a longer time,’ by a less excentric 
umbilicus, and by the outward shelving of its whorls—a feature which also 
gives a deeper edge to the inner margin and a subtriangular appearance to the 
“aperture.” 
1 The carina is distinct at 185 mm. diameter,—beyond that the ventral area is damaged. The 
eventual disappearance of the carina is certain—by analogy with terminals of other series. 
