SONNINIA ATTRITA. 371 
nounced costate stage, or, lastly, the septation—one or more of these features will 
distinguish this species from any other morphological equivalents. 
The present species has two features linking it to nodata, which suggest its 
position as a descendant of that one. First, it possesses the same want of 
symmetry in the superior lateral lobe; and secondly, it has an ill-defined inner 
margin. The inner margin is more defined than in nodata,—that is the course of 
development consequent on less gibbous whorls; but it is not so well defined 
as in the nearest morphological equivalents. 
Sonn. paucinodata occurs in the Concavum-zone of Bradford Abbas. Fig. 7 of 
Pl. XCI shows the side view of the type-specimen, reduced to one-half natural 
size; fig. 8 gives the outline of the front view similarly reduced ; fig. 9 illustrates 
parts of two septa of natural size. ‘To facilitate comparison these figures have 
been placed on the same plate as similar figures of Sonn. quadrifida—the only 
species for which the present one could be mistaken. 
Sonninia artrita, S. Buckman. Plate XC, figs. 4—6, 
Discoidal, compressed, carinate. Whorls nearly smooth, ornamented only with 
obscure, undefined folds. (In the umbilicus a spinous stage persists to a 
diameter of about 25 mm. [umbilical diameter 16 mm.]; about another whorl is 
costate ; then the coste are obsolescent.) Ventral area fairly defined, somewhat 
flattened, divided by a small, ill-defined carina. Inner margin fairly defined, 
sloping. Umbilicus graduate, excentric. Inclusion more than one-half. Whorl- 
section oblong, more than one-fourth indented. Suture-line with an asym- 
metrical superior lateral lobe; the terminal lobule intra-axial, anisosceloid, 
inequicellate ; the lateral lobules not opposite. 
The suture-line of this species possesses a superior lateral lobe with the same 
features which characterise those of paucinodata and nodata—want of symmetry on 
account of the intra-axial position of the anisosceloid terminal lobule. Therefore 
this species may be united with the paucinodata-stock, as a descendant of 
poucinodata; and its ornamentation—or want thereof— exactly conforms to what 
would be expected from the further development of that species. 
In regard to species of other stocks, the spinous centre separates it from most 
of the similar, nearly smooth forms; the suture-line separates it from modesta. 
The suture-line, particularly, distinguishes it from Sonn. papilionacea ; but it also 
differs externally, in the section of its outer whorl, in being less umbilicate and 
less costate—the costz in the early costate stage are nearly twice as distant, and 
yet hardly more prominent. 
