8 GENESIS OF THE ARIETIDA. 
and the embryological divisions proposed by Branco.’ The classification 
given above was necessary in order to introduce our remarks upon the Am- 
monitine, and show clearly why we limited this suborder as defined above ; 
any further discussion would lead us too far away from the immediate objects 
of this memoir. 
NOMENCLATURE OF Sraces or GROWTH AND DECLINE. 
In a paper read before the Boston Society of Natural History, November 
16, 1887, the author discussed the classification of the stages of growth and 
decline, dividing them as follows : — 
1. The earlier stages, embracing the ovum (monoplast, Lankester), the 
monoplacula, and the diploplacula, were considered under one term, Protembryo, 
because of their parallelisms with the single and colonial Protozoa. 
2. The next, or blastula stages, were classified under the head of Mesem- 
bryo, on account of their resemblances to the Mesozoa; the latter being those 
forms usually included in the sub-kingdom of Protozoa, but which have true. 
ova and spermatozoa, and can be therefore separated as one-layered, spherical 
Blastrea, closely parallel with the blastula, and precisely intermediate between 
Protozoa and Metazoa. 
3. The gastrula stages were considered as referable to true Metazoa, and 
were styled accordingly the Metembryo. 
4. The earlier planula or ciliated stages were regarded as indicating a still 
very remote ancestral type, in common with Semper, Lankester, and Balfour, 
and were termed the Neoembryo. 
5. The later ciliated stages— those which show the essential characters of 
the type to which the embryos belong — were classified as the Typembryo; ex. 
the veliger, nauplius, etc. The typembryos were considered as the last of 
embryonic stages, and those which followed were regarded as true larvae on 
account of their more demonstrable connections with well known forms. — It 
was found by applying this classification to the fossil Cephalopoda that the pro- 
toconch of Owen was the shell of a univalve typembryo, which must have been 
a veliger not very widely removed in structure from the similar shells of the 
embryos of Gasteropoda and Pteropoda.’ 
The principal difficulty of the application of this view lies in bringing the 
wrinkled and curious forms which occur upon the apices of some Nautiloids into 
1 Mojsisovics, Med. Triasprovinz; Fischer, Manuel de Conchyliologie ; Zittel, Handbuch der Paleon- 
tologie; Branco, Paleontogr., RVI we VL 
2 Robert Tracy Jackson, a pupil of the author, in an essay now in preparation (‘‘ Phylogeny of the 
Pelycypoda’’), shows that the typembryo stage of mollusks is limited to an early period characterized by 
the existence of a shell-gland and the plate-like beginnings of a shell. Later veliger stages, he says, are ref- 
erable to the class or phylum of Mollusca, to which the embryo really belongs, and he names them ‘ Phyl- 
embryo” stages. The ‘ prodissoconch”’ is a name given by Jackson to the embryonic, bivalvular shell of 
Pelyeypoda, which is the equivalent of the protoconch of cephalous mollusca. The completed protoconch 
of the cephalous mollusca, and prodissoconch of Pelycypoda, Jackson considers as a stage later than that at 
which the phylembryonic characters are emphasized, and as the close of the embryonic shell period. His 
paper will give types of these and other stages considered in the several classes of mollusks. 
