146 EOCENE MOLLUSCA. 
separation is unnecessary. The mammellated apex, which is formed by the large 
development of the shell in embryo, cannot be regarded as a character of generic value; 
in fact, we find, among the recent Volutes, on the one hand, every degree of develop- 
ment between the broadly mammellated apex of Vol. imperialis and the pointed, almost 
mucronate, spire of Vol. Harpa; and among the fossil Volutes, on the other hand, we 
may trace the apex gradually enlarging from the small conical pullus of /. dwctatriv and 
V. spinosa through V. cithara, V. muricina, &c., into the papillary pullus of V. Wetherelli, 
and thence into the mammellated apex of the Crag species, 7. Lamberti. Similar varia- 
tions may be observed in the condition of the columellar plaits. The characters in 
question, therefore, cannot, I think, be regarded as of generic value, however useful 
they may be for the division of the genus into groups. 
Since the genus Volutilithes was proposed, the recent species /. abyssicola has been 
discovered. This Volute, which was dredged off the Cape of Good Hope at the great 
depth of 132 fathoms, presents a very close resemblance to some of the Eocene species, 
and appears to be the sole living representative of the group. 
By far the greater part of the numerous eocene species which belong to this genus, 
are ribbed, or crowned with spines, and transversely furrowed ; and the specific cha- 
racters are taken chiefly from the varied conditions or the absence of these ornaments, 
and also from the condition of the outer lip, and the columellar folds. With regard 
to the specific distinctions which depend on the condition of the ribs and spines, or on 
the transverse sulcation, it must be borne in mind that these characters are subject to 
considerable modification by age; and that in almost all the species, the shells, even 
those which are simple and smooth at maturity, are, in their very early state, ribbed 
and coronated, as well as transversely sulcated. In some species, these ornaments 
disappear after the first three or four whorls ; in others they become gradually more 
or less obliterated as the shells are enlarged; and the ribs in the young shells are, 
almost invariably, more numerous, and extend further over the whorl than in mature 
specimens. The character of the spines is subject to considerable modification, and 
the number of the columellar folds is also liable to slight variation in the same species. 
These characters, therefore, must be used with caution, and with a due regard to the 
apparent age of the specimens. The characters which appear to be the most certain 
are those which depend on the condition of the pullus or embryonic shell, on that of 
the notch at the base of the shell, and on the more or less effuse state of the inner lip. 
The condition of the notch, in case the notch itself is not preserved, will be found 
represented by the ridge at the base of the columella, called the “comb” or 
“crest,’ which is due to the reflexion of the base of the shell caused by the notch, 
and will be more or less prominent, according to the greater or Jess depth of the 
notch. 
The following distribution of the English species into groups is made solely with a 
view to assist the student in the frequently perplexing task of arrangement ; and it is 
