234 STUDIES IN EVOLUTION 



growth, by which nepionic and sometimes neanic features are 

 pushed forward, or appear earlier in the history of the indi- 

 vidual, so as to become impressed upon the early embryonic 

 shell. Only a brief review of these changes -will be noted 

 Jiere, as a fuller description properly belongs under the dis- 

 cussions of the various genera and families. Naturally, the 

 greatest departure from the normal protegulum is exhibited 

 in the most variable and specialized valve, the ventral valve. 

 The nearly equivalve genera, as Lingula and G-lottidia, pre- 

 sent almost no modification. In the ventral valve of Linnars- 

 sonia and Orbiculoidea (Plate XI, figure 7), the protegulum 

 has a hinge more or less arcuate. Discinisca shows a sub- 

 circular ventral protegulum with a pedicle-notch, and the evi- 

 dence of any hinge in the dorsal valve is very slight (Plate XI, 

 figures 8, 9). The discinoid character appearing in the second 

 and third nepionic stage of the Paleozoic Orbiculoidea (Plate 

 XI, figure 6) has become so accelerated in neozoic and recent 

 Discinisca as to produce a discinoid protegulum. 



The strophomenoid shells usually retain a normal protegu- 

 lum in the dorsal valve, but in the ventral valve the pro- 

 tegulum has an abbreviated hinge and arcuate hinge -line 

 (Plate XI, figures 13, 14, 15). 



No marked variation has been yet observed among the spire - 

 bearing genera, nor has any been seen in the terebratuloids 

 or rhynchonelloids further than the radii on the protegulum 

 of Atretia (Cryptopora). Possibly this feature in Atretia is 

 an inheritance from the radiate character of the shell in the 

 Rhynchonellidse. It may be, however, one of the features 

 consequent upon its fragile nature and deep-sea habitat, as 

 observed among other abyssal shells. 



Differences in the Valves. 



The dissimilarity in the form and relations of the two 

 valves progressively increases in the following genera : Lin- 

 gula, Terebratulina, Cistella, Discinisca, Thecidium (Lacazella), 

 and Crania. Lingula is nearly equivalve, both valves bear- 

 ing a close resemblance to each other. In Terebratulina and 



