352 STUDIES IN EVOLUTION 



on the fold and sinus. The long and broad varieties do 

 exist, however, but are of such infrequency as to suggest 

 that they are not genetic variations from the typical form 

 occurring in this locality. 



The specimens from the Wenlock shales of Dudley show 

 a considerable variation from their American congeners in 

 having more numerous plications, of which a greater number 

 is raised on the dorsal fold and depressed in the ventral 

 sinus. In other respects it is believed that the description 

 here given of the development of the shell will apply to the 

 British form. 



Rhynchotreta cuneata, although considered as abundant in 

 the mature state, does not approach, in the number of young 

 specimens, Camarotoechia Whitii, C. neglecta, Q. indianensis, 

 Homoeospira evax, Whitfieldella nitida, Spirifer crispus, Sp. 

 crispus, var. simplex, Atrypa reticularis, Rhipidomella hyhrida, 

 and Dalmanella elegantula. The entire number of young in- 

 dividuals examined is about one hundred and fifty, ranging 

 in size from .8 to 1.5 mm. in length. The mature forms 

 average about 17 mm. in length. 



In the several series selected from the material at hand, it 

 is evident that the shell assumed the characters and form of 

 maturity when reaching a length of about 10 mm. At this 

 period of growth the fold of the dorsal valve becomes ele- 

 vated, and the sinus of the ventral valve depressed (figure 15, 

 Plate XVIII). Previous to this stage, the dorsal valve is 

 depressed and transversely concave, and the plications of the 

 opposite valve are raised along the median line of the shell. 



Specific Characters. 



Mature Form (Plate XVIII, figures 14, 14 a, 14 5, 22, 22 a). 

 — Shell triangular, cuneiform, widest across the pallial region. 

 Length equal to about twice the depth of the valves. Beaks 

 compressed laterally, attenuate and pointed. 



Ventral valve moderately convex, sub-angular along the 

 latera, marked by a deep sinus, which commences near the 



