364 INFERIOR OOLITE AMMONITES. 
compression, and for its very simple suture-line. These characters bring it into 
connection with costata and with submarginata ; but it is more slowly coiled, more 
umbilicate, more numerously ribbed, less graduate in the umbilicus, less lobate, 
and thinner than the first ; while it is less centrally spinous, more coarsely ribbed, 
with a more angular ventral area, and more degraded lobes than the second. 
From euwromphalica it differs in having a smaller, shorter spinous stage, a 
slightly smaller umbilicus, and in being more compressed ; in fact, the changes 
are such as would be expected in retrogressive development by earlier inheritance. 
It is, therefore, presumably the descendant of ewromphalica. Practically it agrees 
with it in suture-line, that is if it be supposed that there has been degeneration in 
the suture-line as well as in other features, and that the lateral lobules of the 
superior lateral lobes have become so aborted as to be detected only with 
difficulty. The degraded type of suture-lime in comparison with that of other 
Sonnime is certainly very noticeable. 
Sonn. omphalica is a somewhat rare form from the Concavum-zone of Bradford 
Abbas. A presumably adult specimen is depicted in Pl. LXXXIII,—side view 
fig. 5 (the spinous stage of its inner whorls has suffered somewhat from the 
tool in cleaning), front view fig. 6, and suture-line fig. 7. The side view of a 
young specimen (fig. 9 of the same plate) illustrates the spinous stages ; its front 
view is shown in fig. 8. 
The quadrifida-stock. 
he species of this stock are noticeable, particularly, for the balance maintained 
in the proportions of the superior lateral lobe. The lobe is, therefore, 
symmetrical ; and the cruciform arrangement is evident. ‘I'he terminal lobule is 
axial,—that is, it forms a continuation of the main axis of the lobe; it is isosceloid, 
—bhbecause the axil on either side is equally distant from the tip;' and it is 
equicellate,—that is, the cells separating it from the inner and outer lobules are of 
equal size and depth. The lateral lobules are equipoised,—that is,they are both much 
of the same size and ‘‘ opposite ”’ (springing from the lobe at points opposite to each 
other); and they are both more or less bipartite. The bipartition becomes most 
noticeable in Sonn. papilionacea, where it causes the two lateral lobules to form 
arude ><. In all the species the terminal lobule is rather long, rather narrow, 
and not much branched. 
1 If, without taking account of the offshoots, straight lines be drawn from each axil to the tip, 
and the two axils be similarly connected, an isosceles triangle is formed, of which the line between 
the axils is the base. 
