324 



A — Smooth stage. 



B — Curved rib stage. 



C — Cancellated stage. 



D — Spiny stage. 



E — Senile stage. 



I- — -Individual variation. 

 The acceleration of the Jackson race is beautifully brought out in 

 this diagram, and as its correlative, the recapitulation in the earlier onto- 

 geny of the later races, of the adult characters of the ancestral race. The 

 individual variations may occur on any part of the shell, but usually fol- 

 low stage C. 1 



Bracliiopoda. — Among the members of this class there is a wealth of 

 illustrations of recapitulation. I can only select a few cases that have 

 been worked out in such a way that the relationships of the forms are in- 

 dicated by the morphological and geological series as well as by the on- 

 togeny. The pioneer student of the correlation of ontogeny and phylogeny 

 among the brachiopods was Beecher, whose refined researches in paleo- 

 biology have never been excelled and rarely equaled. 



The developing brachiopod, in the later embryonic stages, secretes in 

 the mantle on opposite sides of the body two shell plates, which by periph- 

 eral growth ultimately meet at the edges and form the initial shelly 

 investment of the animal. This initial shell to which Beecher has given 

 the name "protegulum" (6) is of very simple form, consisting substan- 

 tially of two convex plates of semicircular plan, gaping at the posterior 

 straight edges. Through this gap between the two valves the pedicle 

 (organ of attachment) projects. At first the pedicle occupies the full 

 width of the valves, but subsequent peripheral growth of the shell with- 



1 For additional studies of the gastropoda from the developmental standpoint 

 see the following: Koken, E., Ueber der Gastropoden voni Cambrinm bis zur Trias.. 

 Jahrh. fiir Mineral. G-eol. u. Pal., 1SS9. Beil. Bd. vi. Linden, Grafen M. von. 

 Die Entwicklung der Skulptur und der Zeichnung bei den Gehausschnecken des 

 Meeres, Zeitschr. Wiss. Zool., vol. Ixi. 1896. Grabau. A. W., Studies of Gastro- 

 poda II, Fulgar and Sycotypus, Am. Nat., vol. xxxvii, 1903 ; Phylogeny of Fusus and 

 its allies, Smithsonian Miscell. Coll., vol. xliv, 1904 : Studies of Gastropoda III 

 on Orthogenetic variation, Am. Nat., vol. xli, 1907. Smith, Burnett, Phylogeny 



of the species of Fulgur with remarks on an abnormal specimen of Fulgur canal- 

 iculatum and sexual dimorphism in Fulgur carica, Proc. Acad. Nat. Sci. Phila., vol. 

 liv, 1902; Senility among Gastropods, Proc. Acad. Xat. Sci. Phila., vol. lvii, 1905: 

 Phylogeny of the races of Volutilithes petrosus, Proc. Acad. Sat. Sci. Phila., March, 

 1906 ; A new species of Athleta and a note on the morphology of Athleta petrosa, 

 Proc. Acad. Xat. Sci. Phila.. May. 1907; A contribution to the morphology of 

 Pyrula, Proc. Acad. Sat. Sci. Phila., May, 1907. 



