116 PSEUDOCERATITES OF THE CRETACEOUS. 
METOICOCERAS* n. gen. Hyatt. 
The young, although distinct from those of Heinzia, enable us to 
connect Heinzia and its more modified allies with what I have misnamed in 
Zittel’s Vext-book the Pedioceratide,’ here corrected to Mantelliceratidee. 
The type species, if the full-grown shells were the sole evidence, 
would necessarily fall into the same genus with Heinzia matura on account 
of the resemblance of the compressed later’ stages. But the following 
description of the young shows this to be a case of morphic equivalence 
based on entirely distinct modes of development. 
The earliest stage observed has a coronate form with large lateral 
tubercles. This stage is of short duration and appears on what seemed to be 
still the nepionic stage at about 2 to 2.6 mm. in diameter. The venter at 
this time is broadly rounded and without coste on the cast. Subsequently 
at diameter of about 3 mm. the ventral rows of tubercles begin to appear 
upon fine coste that pass across the venter and connect the rows of tubercles, 
and the coste also begin to spread inward on the sides. The termina- 
tions of these at 4 mm. diameter were without tubercles. The coste are 
partially bifurcated at the outer row of ventro-lateral nodes, but only one 
of each pair passes across the venter. The two rows of ventral tubercles 
are borne upon this costation, and in front of each one is a depression or 
transverse furrow exactly like those that are so peculiar and characteristic 
of Cheloniceras royerianum. Intermediate coste begin to appear a little later 
and at first are without tubercles and are single. In this stage there is an 
evident modification of a late neanic substage of Cheloniceras, which has 
similar costee but a more coronate form. These costee subsequently form a 
branch of the dichotomous lateral costations or may remain single and 
short. Though the resemblance to Aspidoceras is apparent, the general 
form and aspect at this age can be accounted for as a parallelism with Pseu- 
daspidoceras of India, which is one of the same stock, and arises as the 
result of the assumption of the subquadrate form after the coronate stage 
in related organisms. It is of course very likely that this Aspidoceran 
aspect may have some genetic significance, but the connections that would 
prove this are not as yet clearly made out. One can not accept all of the 
characteristics that appear in larval forms as of unquestionable genetic 
significance. They are safe guides only when carefully compared and 
« Mérozkos, an emigrant. > See Mantelliceratide. 
