C. E. Beecher — Development of the jBrachiopoda. 349 



One of the most conspicuous examples of a difference in the 

 form of the valves is shown in the abnormal genus Probosci- 

 della. In early nealogic stages, it resembles an ordinary 

 Productus. Afterwards, probably from burrowing in the 

 mud, the ventral valve becomes extravagantly developed 

 anteriorly into a calcareous tube. This is accomplished by the 

 excessive growth of the anterior and lateral margins. Then 

 an infolding takes place until the lateral edges unite, after 

 which the tube is built up by concentric increment around the 

 free end. The resemblance of Proboscidella to Aspergillum 

 is quite marked, except that, in the latter genus, the tube is 

 formed from the growth and union of two valves instead of 

 one. 



From the morphological differences of the pedicle and 

 brachial valves, it will be seen that the highest modifications 

 occur in the former; while the variations in the latter are 

 expressed mainly as adaptive reactions or accommodations to 

 these changes. The explanation of the fact that greater alter- 

 ation takes place in the pedicle valve evidently lies not in the 

 greater plasticity of this member, but in its more highly 

 specialized and differentiated external form, and mainly in its 

 being the lower and attached valve. 



]STo account is taken here of the crura, loops, and spires of 

 the brachial valve, so characteristic and important in many 

 families and genera. These are evidently processes developed 

 by the internal requirements of the animal and are not affected 

 by the environment. Therefore, they are internal calcified 

 organs independent of the form or manner of growth of the 

 external covering. This is shown by the fact, that, in 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 hinge. In typical and generalized' 

 forms, as Lingula, Terebratulina, Cistella, and Discinisca, 

 considered as before in regard to length of pedicle, free- 

 dom of movement, and direction of longitudinal axis to 

 the object of support, we find a key to these types of struc- 

 ture. In the individual development of Terebratulina, as 

 shown by Morse, we .first have the early embryonic shell 

 (protegalum), with a short pedicle and straight hinge. The 

 next stage retains both these characters, but the valves have 

 become more unequal and the pedicle opening confined to the 

 fissure of one valve. The result is a shell very much like 

 Argiope or Megerlia (Megathyris and Muhlfeldtia), to which 



