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INFERIOR OOLITE AMMONITES. 



to the decrease of ornament, a feature similar to what may be noted in the case of 



Amaltheus. 



From Hammatoceras, which parallels this genus very closely in retrogressive 

 development — in some species particularly the likeness is very close — Sonninia 

 may be known by the greater forward curvature of its ribs on the ventral area, the 

 lesser proportionate depth of its saddles, and the absence of a strongly-retracted 

 or dependent inner portion of the suture-line. 1 



From Zurcheria the genus Sonninia is distinguished by its carina and the 

 different position of its spines ; from Haplopleuroceras by not having two rows 

 of spines. 



From Dorsetensia the costate and less ornamented forms are separated by 

 their more complex suture-line, with its peculiar cruciform superior lateral lobe. 

 No unispinous species of Dorsetensia have yet been found. 



From Witchellia, in general, the longer lobes with their narrower stems 

 become the chief features distinguishing the present genus ; but other matters will 

 be noticed when that genus is described. 



From " Pleuroceras" and Amaltheus the present genus is easily separable by 

 the absence of a crenulate carina. 



Some of the species which have retrograded so much as to have, practically 

 speaking, lost all distinguishing ornament, become very like certain genera of 

 the Hildoceratidge ; but they may be distinguished therefrom by the peculiar 

 features of their suture-line, namely, the depth of the saddles, the somewhat 

 cruciform arrangement of the superior lateral lobe, and the greater general 

 complexity of the septa. 



Like the other genera of the Amaltheidse, Sonninia must be regarded as a 

 cryptogenetic series at present. The species representing the progressive stages 

 of development (p. 289) which preceded the spinous stage have not yet been 

 discovered. From the fact of the spines being an embryonic feature the existence 

 of a considerable series of spinous forms may be inferred ; because, from 

 analogy with the development of other groups, it may be argued that the 

 spines were once confined to the adult, and that they have become embryonic 

 by the law of earlier inheritance. 2 This alone implies a considerable and varied 

 series of ancestors. Such spinous species would be, presumably, the highest 

 forms of the progressive series, and the nearest approach to them is seen in 

 Sonninia multisp inata. 



1 "Descent of Sonninia,'" &c, ' Quart. Journ. Geol. Soc.,' vol. xlv. 



2 la all the species which have come under my notice the spines are solid, but in the armatus- 

 gruup (Dcroceras) the spines are hollow, that is, separated by a partition in the same way as a hollow 

 carina (see p. 81, foot-note). The difference is most striking in the casts of spines of Sonninia and 

 l)eroceras. 



