940 THE AMERICAN NATURALIST. [VoL. XXXVI. 
determination of the genetic relation of PJatyceras arkonense 
Shimer and Grabau, from the Hamilton of Ontario. 
Natica neriteformis, from the European Muschelkalk (Queen- 
stedt), as far as can be judged from the internal molds which 
are alone represented, is a phylogerontic member of a simple 
naticoid group of shells, Fissurella, Acmza, and other patel- 
loid types, so common in the Mezozoic and modern sea, have, 
as already noted, a coiled protoconch. They therefore repre- 
sent phylogerontic types, in which the power of coiling has 
been lost after the completion of the protoconch stage. In 
Crepidula the power to coil is still retained in a slight degree. 
In the majority of specialized gastropods phylogerontism is 
expressed, not in the non-coiling of the last portion of the 
spire, but in its expansion and wrapping about the earlier 
whorls so that these become more or less concealed. This 
is generally accompanied by the loss of all ornamental char- 
acters except, in some cases, the coloration. Sometimes it 
happens, however, that spines arise independently on this 
portion of the shell. This is most marked in Melongena and 
can be readily explained by the peculiar manner in which the 
shell develops. | 
In the young stage every typical Melongena has the char- 
acters of Hemifusus. In accelerated species of this genus 
spines follow the tubercles, the two grading into each other as 
in Fulgur carica. In other species, however, notably Z. colosseus 
Lam. and H. (Melongena) pugilinus, a keeled or even smoothly 
rounded stage intervenes between the tubercled and spinous 
stages. (See ante.) This enables us to understand Melongena.? 
The two typical species of this genus appear to have been 
derived from the same ancestral species, which coexists with 
1Shimer, H. W., and Grabau, A. W. The Hamilton Group of Thedford, 
Ontario, Bull. Geol. Soc. Am., vol. xiii, p. 176. 
? Only the two typical species, Melongena melongena and M. patula, are referred 
to here. Most of the other species generally referred to this genus belong else- 
. where. Such species as M. morio and M. Pugilina are transitional from typical 
Hemifusus, eg., Æ. colosseus Lam., being produced chiefly by a condensation 
of the typical Hemifusus characters. They are generally classed with Melongena, 
. but have not the typical phylogerontic growth of that genus. ‘They will be 
referred to under both names. 
