238 STUDIES IN EVOLUTION 



each group there is a frequent recurrence of similar general 

 external features, whether in crurate, looped, or spire-bearing 

 genera. 



Genesis of Form. 



The principal characters shared by the two valves are the 

 general outline and the liinge. In typical and generalized 

 forms, as Lingula., Terehratulina^ Cistella,, and Discinisca, 

 considered as before in regard to length of pedicle, freedom 

 of movement, and direction of longitudinal axis to the object 

 of support, we find a key to these types of structure. In the 

 individual development of TerebratuUna, as shown by Morse, 

 we first have the early embryonic shell (protegulum), with 

 a short pedicle and straight hinge. The next stage retains 

 both these characters, but the valves have become more un- 

 equal and the pedicle-opening confined to the delthyrium of 

 one valve. The result is a shell very much like Argiope or 

 Megerlia (Megathgris and MuUfeldtia), to which Professor 

 Morse also called attention. The same author next showed 

 that the succeeding stage had a comparatively long pedicle, 

 and a shell linguloid in form. Afterward the defining of 

 the pedicle-opening, shortening of the pedicle, and truncation 

 of the ventral beak produced the final characteristic external 

 features of TerebratuUna. The deduction from this example 

 and from Lingula is that genera having pedicles sufficiently 

 long to admit of freedom of axial movement have elongate 

 and rostrate shells. The shortening of the pedicle brings the 

 posterior part of the shell in more or less close proximity to 

 the object of support, and, as growth cannot take place in 

 that direction, it increases laterally, resulting in broader 

 forms with extended hinge-areas, as in many species of 

 Cktella., Seenidium^ Milhlfeldtia, Terebratella, Kraussina, etc. 



The variety known as Milhlfeldtia truncata, var. monstruosa 

 Davidson, further shows how discinoid characters may be 

 produced in an entirely different type of shell. A specimen 

 was found by the writer in a position which readily gave the 

 solution to its variation from the normal species. It was 



