SONNINIA COSTIGERA. 429 



More gibbous-sided whorls, a more concentric umbilicus, a slower manner 

 of coiling, a less definite inner margin, and smaller, closer set costae in the 

 early costate stage distinguish this form from Sonn. maryinata. The charac- 

 ters of the margin, the coiling, and the costae separate it from Sonn. regula/ris ; 

 and, further, the septation distinguishes it from either of these species. Less 

 reclinate costas and a nearly symmetrical superior lateral lobe mark it as different 

 from Sonn. ptycta or Sonn. cymatera. 



The species to which Sonn. costigera has the most resemblance is really Sonn. 

 obtasiformis, although the much stronger ribbing appearing at an earlier age in 

 that fossil is a noticeable feature of contrast. However, the mode of coiling is 

 the same, and the enlargement of the ephebic ribs is similar though not so 

 pronounced. The suture-line is also similar to a certain degree — there is a 

 resemblance in the bipartition of the inner lobule of the superior lateral lobe ; 

 while the points of difference — the intra-axial position of the terminal lobule, and 

 the bipartition of the outer lobule in Sonn. obtusiformis — may be features 

 correlated with the further development which the costae indicate. The figured 

 specimen of Sonn. costigera was alluded to in p. 333 as probably intermediate 

 between Sonn. ptycta and Sonn. cymatera; but the subsequent removal of matrix, 

 and after much labour the exposure of part of the suture-line, indicates that this 

 idea cannot be sustained. Yet its association with obtusiformis alluded to in the 

 same page — it is spoken of as indicating the connection of ptycta and obtusiformis 

 — is apparently correct, as may be inferred from my remarks above. It is 

 necessary, then, to remove costigera and obtusiformis from association with the 

 acanthodes-stock ; for they do not show that peculiar abbreviation of the inner 

 lateral lobule of the superior lateral lobe which is seen to such perfection in Sonn. 

 cymatera — especially in the adult, Plate C, fig. 6, — nor such reclinate ribbing. Then 

 it becomes desirable to know whence Sonn. costigera may have sprung ; and the 

 only species which can be suggested at present is Sonn. locuples (see later, p. 431). 

 In that species, however, there is a strongly-lobed suture-line with very lobulate 

 lobes. The lobation and the lobulation are much less pronounced in Sonn. 

 costigera, and still less so in Sonn. obtusiformis ; but decline in such respects might 

 certainly have accompanied increase in costation — a much corrugated test would 

 of itself be stronger than one which was plainer, and it would therefore need less 

 support. Septal degeneration, then, cannot be regarded as a difficulty ; but the 

 intra-axial position of the terminal lobule of L in Sonn. locuples, and its axial posi- 

 tion in Sonn. costigera may be. It is an allowable surmise that the ancestor of Sonn. 

 costigera was similar to Sonn. locuples, but possessed of an axial terminal lobule to 

 L, and possibly of less ramified septa. On these grounds the present position of 

 costigera and obtusiformis, near the locuples-group, is fairly well supported, 

 although considered by itself alone the very slight intra-axial position of the 



