8 GENESIS OF THE ARIETIMS. 



and the embryological divisions proposed 'by Branco. 1 The classification 

 given above was necessary in order to introduce our remarks upon the Am- 

 nionitina?, 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 Stages of 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 trie 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. 2 



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 Mojsisovies, Med. Triasprovinz; Fischer, Manuel de Conchyliologie ; Zittel, Handbuch der Paleon- 

 tologie; Branco, Paleontogr., XXVI., XXVII. 



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 

 Pelycypoda, 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. 



