360 INFERIOR OOLITE AMMONITES. 
area, expand the sides, and so become depressed obtriangular.' Compression of 
the sides produces, later on, a peroval whorl-section (Pl. XLIX, fig. 9), and 
further compression produces the oval “‘ aperture’”’ of the adult. 
Sonninza DecoraTA, S. Buckman. Plate LXXXIV, figs. 7, 8. 
Discoidal, compressed, hollow-carinate. Whorls ornamented in the brephic 
stage with regularly-set spines, in the neanic stage with reclining ribs, of which, 
at first, about every third, later, about every fifth, carries a spine; in the ephebic 
stage with reclining, direct, ventrally-inclined ribs, of which, at first, about every 
fifth carries a spine—the rib breaking into two or three outside the spine; later, 
every second or third rib is slightly nodate at its middle, sometimes bifurcating 
beyond; and, later still, there are irregular-sized ribs with occasional obscure 
bifurcation. Ventral area subacutely arched, divided by a small, well-defined 
hollow carina. Inner margin ill-defined, subconvex. Inclusion about two-fifths. 
Umbilicus open, at first regularly, later irregularly spinous. Whorl-section ovate. 
It is assumed that the specimen figured is adult. In relation to Sonn. 
subspinosa, it stands just one remove in accordance with earlier inheritance and 
retrogressive development; it is distinguished from that species by its thinner 
form and the early decadence of the spines. It is unfortunate that I cannot give 
the suture-line of this species, but none of my specimens show it. 
Sonn. decorata, though somewhat differently ornamented, has a general resem- 
blance to Sonn. submarginata, but it is a much thinner shell with a more acute 
ventral area. The sharper ventral area is important—it indicates greater retro- 
gression, either phyletic or ontogenetic; it is, in fact, a gerontic or a phylogerontic 
character. In other words, in retrogressive development the periphery becomes 
more acute with age in the individual or in the race owing to whorl-compression. 
Therefore the flatter-abdomened but older suwbmargimata could not be the adult of 
the more acute-abdomened decorata. 
The Concavum-zone of Bradford Abbas, Dorset, has yielded the examples in 
my cabinet. It is presumably not a rare species, though possibly somewhat 
uncommon. ‘T'wo views, side and front, of this species are given in Pl. LXXXIV, 
figs: ¢, 0. 
! Inverted triangle—the narrowest part towards the centre of the shell. 
