300 STUDIES IN EVOLUTION 



that their equivalent stages fall into parallel lines with 

 those of Magellania and Macandrevia* 



The line begins in an early larval stage (Plate XIV, 

 figure A), in which there is a simple circlet of tentacles 

 without a calcified loop, a structure comparable in every 

 respect with Gwynia (figure Aa). 



The next stage (Plate XIV, figm-e B) shows the growth of 

 a septum inflecting the line of tentacles, and producing an 

 arrangement of parts similar to Cistella, although the loop is 

 not calcified. The completion of this structure results in 

 Cistella (figure Bi), and specimens having the characters 

 presented by figures B, Bi, are referred to the cistelliform 

 stage. A fossil representative of this type is Zellania, from 

 the Jurassic (figure Ba). 



Megathyris (figure B2) offers neai-ly the same structure as 

 Cistella^ but the growth of two lateral septa or projections 

 has produced two additional inflections in the loop. Tliis 

 completes the line of development in the MegathyriucB. 



Next, considering the Dallinince in their ontogeny and 

 morphology, it is found that after passing through the 

 gwyniform and cistelliform stages (Plate XIV, figures A, B) a 

 form like Platidia is reached (figure Ci), of which Platidia is 

 the living adult representative (figure C-i). The platidiform 

 stage is sho\vn in Macandrevia cranium (figures Ci, Di) ; 

 Dallina septigera (figure D2) ; D. Jloridaiia (figure C3) ; and 

 MiXhlfeldtia sanguinea (figures C2, Ds). 



The growth of the secondary loop on the septum and the 

 subsequent partial resorption produces a structure (1) like 

 that in the fossil genus Ismenia, and (2) one identical with 

 that of adult MiiMfeldtia (illf. truncata and M. sanguinea^. 

 In Plate XIV, figures E1-E5 show the ismeniform stage of 

 Macandrevia^ Dallina, and MilhJfeldtia, as well as the final con- 

 dition of Ismenia^jf and figures F1-F4 represent the milhlfeldti- 



* The illustrations on Plate XIV are taken from Davidson,^.^ Fischer and 

 CEhlert,^ Deslongcliamps,^ and Friele.^i with original drawings by the writer. 



t Figures Fc and F^, Plate XIV, are from Davidson.^ They are Jurassic 

 species, and were referred to Terebratella (T. furcata Sowerby, figure Fs, and 



