348 C. K Beecher — Development of the Brachiojpoda. 



object of support, and the animal is capable of raising and 

 rotating it only to a slight degree. Under snch circumstances, 

 the lower valve is wholly different in its relations to the 

 environment, and, naturally, it expresses the greatest dissimi- 

 larity in the two valves of any genus yet discussed. In some 

 allied genera, as Discina (type D. striata) and Schizotreta, 

 where the pedicle is small and the lower valve rises' above the 

 object of support, a similar form in both valves is again pro- 

 duced by the conical growth of the lower valve. 



More primitive types, as Acrotreta and Acrothele, having 

 the plane of the brachial valve at right angles to the direction 

 of the pedicle, retain a marginal upper beak, while the lower 

 is elevated, subcentral, and perforate. These features in 

 Acrotreta and Discina resemble, in a measure, those in the 

 rudistes. In Acrotreta as in Caprotina, the upper valve shows 

 its normal affinities, while the other has become highly 

 modified and dissimilar. But in Discina and Hippurites, the 

 hinge line is lost, and the apex of the upper valve is subcentral. 

 This conical habit of growth in erect attached organisms has 

 been explained as the physiological reaction from equal radial 

 exposure to the environment. It constitutes the law of radial 

 symmetry, ably discussed by Haeckel, Jackson, Korshelt, and 

 Heider. Its application to the Brachiopoda can be made 

 mainly in forms having the pedicle perforation subcentrally 

 located in the lower valve. 



In Thecidium and Crania, the calcareous union of the lower 

 valve to the object of support represents the extreme of unlike 

 conditioning, and such forms exhibit the greatest difference in 

 the features of the opposite valves. Crania being probably de- 

 rived from discinoid stock is without proper hinge. In the his- 

 tory of its development, so far as known, it does not show beyond 

 the protegulum, an early hinged condition. Hence there is 

 no indication of direct derivation from hinged forms. A 

 false hinge is sometimes present, but it clearly shows a second- 

 ary mechanical adaptation, and not a phylogenetic character. 

 On the other hand, true hinged attached genera, such as 

 Thecidium (Lacazella), Davidsonia, and Strophalosia, possess 

 this feature as a later ancestral character, and, in their chrono- 

 logical history, tend to shorten and gradually eliminate it. An 

 illustration of this is seen in the succession of the species in 

 Strophalosia, or in the ontogeny of one of the Permian species. 

 Strophalosia Goldfussi, in early nealogic stages, has a hinge 

 line about equal to the width of the shell, but in mature 

 individuals, it is usually less than one-half the width. This 

 reduction of the hinge and ostrean form of growth are in 

 accordance with the deductions and observations made upon 

 the Oyster and its allies by Jackson, and the mechanical prin- 

 ciples are evidently the same in both cases. 



