396 STUDIES IN EVOLUTION 



certain what the inceptive condition of this apparatus may 

 have heen. In the early stage represented on page 365, 

 where the ribbon has completed two revolutions, the sup- 

 ports must have been exceedingly tenuous and delicate, for 

 they can be traced in the crystalline or muddy filling of the 

 shell only by extremely faint lines, composed of minute dots 

 of pyrite. As observed under the discussion of these features, 

 the character or actual existence of the loop connecting the 

 spirals was not established, but it is developed, with all 

 normal characters, in a shell 4 mm. in length, where the 

 ribbon makes four revolutions. 



It has been shown by Morse,* that in Terehratulina septen- 

 trionalis the loop (^. e., the entire brachial support) begins 

 by the development of two acute processes from the lower 

 moiety of the dental plate, which assume the character of 

 crura, eventually meeting and coalescing on the dorsal side, 

 forming the completed loop at an early stage, the ventral 

 horns of the loop never uniting. The simple nature of the 

 support in these shells precludes the possibility of the con- 

 tinued growth which obtains in the more complicated appara- 

 tus of the spiriferous species. The inception of the brachial 

 support was observed by Morse in an individual 1 mm. in 

 length, but the lateral processes are not conspicuously devel- 

 oped until a length of 3 mm. is attained, and they have not 

 united at a length of 4 mm. It is therefore possible, from 

 these data, that Homoeospira evax does not have the loop 

 completed at so early an age as that indicated by a length 

 of 2.5 mm. 



The observations by Morse are corroborated by those of 

 Dall f on Leiotliyrina cuhensis. 



Surface Ornaments. — Nearly all the observations upon 

 initial shells or upon that portion situated at the ai)ex of the 

 beak of more advanced stages and representing the initial 



* On tlie Early Stages of Terehratulina sejitentrionalis. Mem. Boston Soc.Nat. 

 Hist., II, pi. ii, figs. 48-55, 1869. 



t Report ou the Bracliiupoda of Alaska ami the adjacent Shores of North- 

 west America. Proc. Acad. Nat. Sci. Phila., 1877, pt. ii, p. 155. 



