284 
INFERIOR OOLITE AMMONITES. 
genera of the Polymorphidse, exhibit most of the changes which are to be found 
spread over the two families Arietidse and EAldoceratidx ; and the changes occur 
in the same successional order. The ventral area is crossed by ribs, is sulcate, is 
obtuse and carinate, is carinate and sulcate, is acute and carinate ; and the shape 
of the whorls varies from gibbous and very evolute to compressed involute. The 
gradations between all these various stages are to be found among the species 
described. 
I have given a table, pp. 282, 283, to show the genealogical connection of the 
various species of certain genera of the Polymorphidse. This illustrates what 
I have frequently pointed out in the text, that the descent of the Polymorphidse 
differs materially from the descent of the Hildoceratidss. I have also indicated in 
the text the various gradations by which the different species have been pro- 
duced, and how their genealogy has been arrived at. This difference of descent is 
a matter most important to establish on account of the great likeness between 
Bumortieria and Grammoceras. 
It will be seen from the table that there is one main radical Polymorphites 
derived from Polymorphites poly morphus of the Jamesoni-zone. It produces through 
Pol. Vernosse a degenerate descendant showing all the characters of its ancestor, 
but also the compressed quick-coiled whorls indicative of senility. Pol. senescens 
and Pol. norma are also the latest species with which I am acquainted exhibiting 
the primitive form of abdomen known as the " jJZamcos^a?^." 
The genus Bumortieria arose from Pol. Vernosse. There is only one link 
missing between this species and Bum. prisca, namely, a species with coarser ribs 
than Pol. Vernosse^ with or without a carina (p. 232). All the Bumortieriae 
are various modifications of Bum. prisca ; and it may be said that after Bum. 
prisca the period of degeneration — as expressed in more quickly-coiled whorls, 
greater compression, and decay of ribbing — commenced in the different series, 
in a greater or less degree, culminating with Bum. Moorei. But, just at the 
commencement of what may be called the degenerating period the size of the 
species was not influenced adversely — in fact it was rather the other way, for 
Bum. pseudoradiosa. Bum. striatidocostata, and most remarkable of all the very 
late-appearing Bum. grammoceroides are all species of large size for this genus. 
The "degenerating" or "senescent and senile period" among the Buraor- 
tierise is especially noticeable for the manner in which the species converge towards 
Grammoceras. The Bumortieria become senile without having attained to the 
sulcate-carinate abdomen which appears in Grammoceras, and also in Catulloceras ; 
but they become senile in the same way as Grammoceras, that is to say, gene- 
rally, continued compression of the whorl causes greater involution and a sharper 
ventral area. The continuation of these processes in the two genera produces 
ultimately two species of extreme similarity, namely Grammoceras mactra and 
