
926 THE AMERICAN NATURALIST.  [Vor. XXXVI. 
should naturally expect, the ribbed were derived from the 
smooth species which precede them immediately in time. The 
immature specimens of H. pyrene illustrated by Ulrich show 
well the faint beginning of undulations which become pro- 
nounced only in the adult. 
A somewhat more specialized ribbed shell from another 
phylum is found in Nazca nexicosta Phillips of the Devonian. 
This in form is truly naticoid, with the ribs sharp, narrow, and 
uniform, separated by uniform intervals which are much wider 
than the ribs. The earliest stages, as far as can be judged 
from the illustration,! are ribless. The whole aspect of the 
last whorl of this shell is strikingly like the final whorls of the 
protoconch of highly specialized Tertiary and recent gastro- 
pods. Natica subcostata Schl. of the Devonian (Stringo- 
cephalen Kalk) of Paffrath, Germany, is another example, in 
which the costz or ribs have become compound, secondary 
shorter ones being intercalated between the longer ones. Natica 
costata from the Lower Trias of St. Cassian carries the simple 
type of costz into the Mezozoic era, while JV. armata from the 
Upper Trias of that region shows the further complication of 
revolving spirals which are cancellated by the sharp ribs. In 
the later forms generally the umbilicus is closed. A similar 
succession of smooth, simple-ribbed, and cancellated-ribbed 
shells appears in Jurassic Nerita and Neritopsis, as illustrated 
by Hudleston in his monograph of the “ Inferior Oólite Gastro- 
poda."? Among the turreted types simple ribs appear in some 
of the Devonian Loxonemas, some species of this genus show- 
ing the further specialization of cancellating spiral lines. 
Similar successions may be found in a great many other series, 
and it is perhaps not too much to say that in the majority of the 
larger phyletic series, except those highly specialized, the radicle 
is a smooth, round-whorled form, succeeded by types in which 
the adults are ribbed, and later cancellated, after which pro- 
gressive modification may be carried further. The index to the 
history of the phylum is in general to be found in the life 
history of the individual member under consideration as revealed 
1 Whidborne. Devonian Fauna of the South of England, Pa/eontogr. Soc. 
Mon., Pl. XIX, Fig. r. 2 Palzontogr. Soc. Mon., Pl. XXVII. 
