PIIYLOGENY OF THE PELECYPODA. 291 



3fesemhryo. The holloAV bla.stula stage comparable to the condition found in the 

 adults of A^olvox and Eiidoiina types of the Mesozoa.' Examples: PI. xxiii, figs. 1,1a, 

 of developing Patella. 



Mctemhryo. The gastrnla, comparable to the lower Porifera in which three cell-layers 

 exist, as in the lowest Hydrozoa. Examples: PI. xxiir, fig. 2, of developing Paludina, 

 also Hatschek's figure 5, of develoi)ing Teredo. 



Ii(foemhnjo. The trochosphere. Stages notyet possessing the essential diagnostic char- 

 acters of the mollusca; comparable to the embryos of Chretopod worms and other Coelo- 

 mata. Examples: PL xxiir, figs. 3-1, of developing Patella, also Professor Butschli's 

 figure (3, of developing Paludina. 



Tijprmhrijo. The period at which an essential molluscan featui-e, the shell-gland, and 

 plate-like beginuing of the shell are discoverable, and yet the embryo is not referable to 

 the class to which it properly belongs. Examples: fig. 22, p. 297, of developing Ostrea; 

 PI. xxiu,fig. 5, of developing Pisidum ; PI. xxru, fig. 6, of developing Pleurobranchidium, 

 also Lankester's (44), fig. 72, of developing Limnseus. A comparison of the molluscan 

 typembryo and neoembryo stages with Hatschek's figs. y6-.53, of a developing Enpo- 

 niatus (Serpula) shows graphically the value of these stages in classification. 



Phylevibryo. Early veliger stages in which the characters of the class subdivisions are 

 indicated, and the structure of the shell and other features render the embrj^o referable 

 to the group of mollusca to which it belongs. Examples : p. 299, figs. 23-24, of developing 

 Ostrea; PI. xxni, fig. 19, of developing Sycotopus and PI. xxiir, fig. 13, of developing 

 Dentalium, also Lankester's (43), PI. 8, fig. 22, of developing Apysia, and om* figs. 

 7-11, PI. XXIII, of developing Anodon, Modiola, Cardium and Montacuta. 



Completed jrrotoconcli and prodissoconcli. The completed first-formed shell of molluscs, 

 the ovisac, fry shell, embryonic shell and larval shell of authors. It finds its representative 

 in the globular, cup-shaped, or spirally coiled protoconch of Cephalopods. Examjiles: PL 

 XXIII, figs. 15, 16, of develo])ing Goniatcs. Also see Professor Hyatt's (27) figures of 

 developing Ammonites and Zittel's figures of young Ammonites, Belemnitcs and Spirula. 

 The existence of a cieati'ix at the base of the conch in Nautiloids is claimed by Profes- 

 sor Hyatt (27) as jiroof that a protoconch existed in the 3'oung and a parallel fact in 

 favor of this conclusion is the finding of a cicatrix at the tip of the conch in Aemfea as 

 described below. 



A withered, irregular bullj, found at the apex of the conch in some Orthoceratites, 

 figs. 1-8, Professor Hyatt considers as the remains of a protoconch, probably originally 

 of conchyolin structure, M. Barrande (3) figures cicatrices and withered bulb-like 

 l)rotoconchs in many Nautiloids. See his figures of Ceplialopoda, Pis. 487-490, inclu- 

 sive. Several of his figures are taken from unpublished drawings of Professor Hyatt's. 



In some Pteropods a bulb-like expansion of the tip of the shell seems to be the equiv- 

 alent of the protoconch ni other mollusks. Examples: PL xxiii, fig. 17, also several 

 figures in Bronn's Thier-reichs. In well-preserved Tentaculites a globular or elongate 

 protoconch is sometimes found. Examples: Tentaciditesacuarius,V\. xxni,^^. 18; also 

 see figures in Hall's PalfBontology of New York and other authoi-s. 



' For tlie consUk'ratiDii of Volvox and Emiorina as mesozoa see Hyatt, Rcfeiviicc No. 30. 



