﻿SONNINIA MAGNISPINATA. 



341 



costate shape of the young is drawn — side view fig. 2, and front view fig. 3. It 

 possesses the test in a very complete state of preservation, and shows, in its inner 

 whorls, spines of the most rudimentary description. It is very instructive to 

 compare this example with spinous forms of the genus, and to note the advance 

 of the costate stage in regard to earlier inheritance. 



The n iagi lispin at a- stock. 



The forms which comprise this series are distinguished by possessing notice- 

 ably reclining ribs. This feature brings them into connection with the acanthodes- 

 stock ; but they are separable therefrom by being more compressed in proportion 

 to development. The reclining ribs which give the shells a twisted appearance 

 prevent their being confounded with species of those stocks which are charac- 

 terised by upright or nearly upright ribs. 



From one another the forms of the series may be easily known, thus : magni- 

 spinata, an elaborate spinous stage in adolescence; semispinata, mostly costate, 

 costad well marked ; brevispinata, mostly costate, costas feebly developed. 



As regards the derivation of the group, the size of the spines in magnispinata 

 shows that a considerable distance separates it from multispinata — in fact, only 

 the central whorls can be considered the morphological equivalent thereof. It is 

 possible for magnispinata to have come from multispinata direct, through a series 

 of forms which constantly enlarged their spines ; or there may have been an 

 intervening series of irregularly-spinous forms — the morphological equivalents of 

 crassispinata a ; and all traces of them may have disappeared owing to the law of 

 "elimination of dissimilar stages," which appears to be the case in regard to 

 acanthodes a. Certain differences in ribbing and shape of whorls in the costate 

 stage prevent magnispinata being regarded as the descendant of acanthodes a ; but 

 there cannot be much mistake in thinking that the two forms are by no means 

 distant relations, sprung from a common spinous stock. 



Sonninia magnispinata, 8. Buckman. Plate LXXVI, figs. 1 — 6. 



1881. Hahpocekas Sowehbti, S. Buckman (non Miller). Inf. Ool. Ammonites; 



Quart. Journ. Geol. Soc, vol. xxxvii, p. 602 

 (pars). 



Discoidal, compressed, hollow-carinate. Whorls, iu section, subcircular to 

 elliptical, and to almost oblong, 1 ornamented with large, regular spines, later with 

 1 This indicates the changes which can be seen, and their order ; the subcircular whorl gradually 



