﻿SONNINIA SIMPLEX. 



327 



present) is quite rounded (in the example figured at a diameter of 145 mm.), 

 but there is a very faint keel-mark on the core. Inner margin well-defined, 

 deep, upright, flat. Inclusion varying with age, the umbilicus markedly excentric. 

 Suture-line with a superior lateral lobe longer than the siphonal, unsym- 

 metrically trifurcate on account of development of the outer lobule ; the inferior 

 lateral and auxiliary lobes fairly symmetrically trifurcate, their lobules divergent, 

 giving them a cruciform appearance ; the inner part of suture-line little dependent. 



This species is an example of very pronounced retrogression. If there be 

 any spines or ribs they are of the most rudimentary description, and they would 

 only be observable in the very centre of exceptionally preserved specimens. None 

 have been detected. 



Sonn. simplex is an extremely retrogressive development of modesta a, with 

 which it is absolutely connected by examples in my possession. Yet this form is 

 perfectly distinguished from modesta by almost complete absence of keel, entire 

 want of ornament, and by its more excentric umbilicus more occluded centrally. 

 The difference in the coiling of the umbilicus in the two forms is the character 

 most likely to attract attention, and it is best illustrated thus : 



modesta. simplex. 



Total diameter measured off .180 mm. ... 180 mm. 



Umbilicus . . 59 ,, ... 60 „ 



,, 1 whorl back . . 25 ,, ... 21 „ 



,, 2 whorls back . . 12 ,, ... 8 ,, 



This excentric umbilicus of Sown, simplex is a feature worth attention. In 

 the development of the genetic series I have noted the tendency to contract the 

 umbilicus ; and this tendency can be traced throughout. But, first noticeable in 

 aged dominans as an adult feature, is a tendency to widen the umbilicus ; and this 

 tendency is also traceable onwards, appearing sooner in each form — in dominans 

 at about 160 mm., in modesta about 120, in simplex about 90 mm. diameter. 

 The tendency to widen is thus preceded by the tendency to contract ; so that the 

 higher the form in the genetic series the smaller the umbilicus in the inner 

 whorls. The tendency to widen has therefore to make up, so to speak, more and 

 more leeway the higher the form. Part of this leeway is made up by beginning 

 earlier the higher the form ; part by the greater and greater tendency to be more 

 umbilicate in maturity. Thus the umbilicus tends to become in the higher, or 

 rather the most retrogressive, forms more and more excentric (see p. 311). 



Sonn. simplex is the terminal of the present genetic series, the entire loss 

 of the carina being an extremely retrogressive feature. As yet the suture-line 

 is considerably complicated — it has, in fact, been increasing in complication as 



