PHYLOGENY OF THE PELECYPODA. 291 



Mesemhryo. The liollow blastiila stage comparable to the condition found in the 

 adults of Volvox and Eudorina types of the Mesozoa.^ Examples: PI. xxni, figs. 1, la, 

 of developing Patella. 



Metembryo. The gastrula, comparable to the lower Poi-ifera 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 developing Teredo. 



Neoembryo. The trochosphei'e. Stages not yet possessing the essential diagnostic char- 

 acters of the mollusca; comparable to the embryos of Chfetopod worms and other Cffilo- 

 mata. Examples: PL xxrrr, figs. 3-4, of developing Patella, also Professor Butschli's 

 figure 6, of developing Paludina. 



Tyjif-nibryo. The period at which an essential moUuscan feature, the shell-gland, and 

 plate-like beginning of the shell are discoverable, and yet tlie embryo is not I'eferable to 

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

 Pl.xxiii,fig. 5,of develojiing Pisidum; PI. xxiii, fig. 6, of developing Pleurobi-anchidium, 

 also Laidvcster's (44), fig. 72, of dcA'eloping Limnsens. A comparison of the molluscan 

 typembryo and neoembryo stages with Hatschek's figs. 36-53, of a developing Eupo- 

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



Phylenibryo. Eai-ly veliger stages in which the chai'acters of the class subdivisions are 

 indicated, and the structure of the shell and other features rendei" the embr3-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 develoj)ing Sycotopus and PI. xxiir, fig. 13, of developing 

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

 7-11, PI. XXIII, of develoi)ing Anodon, Modiola, Cardium and Montacuta. 



Cotn'pletod protocondi and prodissocoiich. 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 pi'otoconch of Cej)halopods. Examples : PI. 

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

 developing Ammonites and Zittel's figures of young Ammonites, Bclemnites and Spii'ula. 

 The existence of a cicatrix at the base of the couch in Xautiloids is claimed by Profes- 

 sor Hyatt (27) as ]iroof that a protoconch existed in the young and a parallel fact in 

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

 descri1)ed below. 



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

 figs. 1-8, Professor Hyatt considers as the I'emains of a protoconch, proljably originally 

 of conchyolin structui-e. M. Barrande (3) figures cicatrices and Avithered bulb-like 

 protoconchs in many Nautiloids. See his figures of Cephalopoda, 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 m other mollusks. Examples: PI. xxiu, fig. 17, also several 

 figures in Bronn's Thier-reichs. In Avell-preserved Tentaculites a globulai- or elongate 

 protoconch is sometimes found. Examples: Tentaculites acuarius,V\. xxiii,fi^. 18; also 

 see figures in Hall's Pal;eontology of New York and other authors. 



' For Uic consklci'atioii of Volvox ami Eiulorina as mesozoa see Ih'att, Reference Xo. oO. 



