﻿SONNINIA ACANTHODES. 



319 



whorl, and continue for about three-fourths of its length. The last two spines 

 shown are separated by a rib, and this kind of ornamentation is continued on the 

 other side, ten ribs bearing only four spines, a spine being last. The bifurcate 

 ribbing demands notice in connection with later species. 



Fig. 3, PI. LXV, gives a side view of crassispinata y reduced one-half, showing 

 regular, succeeding irregular spines ; and fig. 4 is a section of the whorl of 

 natural size. Fig. 5 of the same plate is a section of the whorl of crassispinata /3 

 at the same diameter. 



Sonninia acanthodes, 8. Buchman. Plates LVIII — LX ; LXIII, fig. 1. 



1889. Sonninia acanthodes, S. Buchman. Descent of Sonninia, &c, Quart. 



Journ. Geol. Soc, vol. xlv, p. 658 (pars) (not 

 pi. xxii, figs. 6, 7, 22, 23). 



Discoidal, evolute, hollow-carinate. "Whorls subcircular till nearly mature, 

 ornamented with arcuate, reclining, ventrally-inclined ribs, carrying strong 

 median spines, at first fairly regularly, later irregularly placed. In maturity 

 whorls becoming more subquadrate, the spines replaced by obtuse, elongate 

 knobs, and finally the arcuate ribs are left plain, the ribs being better defined, 

 especially on the inner area, as the spines decrease in size. Ventral area broad, 

 somewhat flattened, the test having a small hollow carina, bordered each side by 

 a shallow depression ; the core, slightly carinate, and the depressions more 

 marked. Inner margin not defined till late, then rather flat. Inclusion barely 

 one-fourth, leaving a space between the spines and the succeeding whorl. 



When I described this species in the ' Quarterly Journal ' I gave it a wide 

 range. The description then given of the immature stage was taken from what I 

 now separate as crassispinata a (p. 318) ; but the remarks on the change which 

 the mature shell showed were made from observation of what I have had for 

 some years labelled as the type-specimen, the example now depicted in PI. LX. 



Two forms of Sonu. acanthodes are figured : 



«. The type, an evolute form with rounded whorls (PI. LX). 



/3. Not so evolute as «, the whorls rather more compressed, the spines lasting 

 longer, the ribs stronger (Pis. LVIII, LIX, fig. 1). 



The differences between the two forms are — « has retrogressed more as 

 regards the spines ; /3 less as regards spines, but more as regards involution and 

 compression. 



The two forms are really cousins derived from multispinata, and they may 

 have come through an adult form, of which the adolescent crassispinata a is the 



