1 30 Gum in gs — Morph agenesis of Platystrophta. 



from indirect evidence.* Davidsonia of the Devonian has 

 strongly impressed spiral markings in the interior of the ventral 

 valve, indicating coiled brachia of several turns. In Leptcena 

 similar evidence indicates still fewer turns to the brachia.f 

 Rajmssquina Jukesi\ shows similar impressions of one or two 

 whorls in the spires. Quite similar impressions are sometimes 

 seen in Product us.§ Among the Orthidce there is no such 

 direct evidence of the position and extent of the brachia; but 

 it cannot be supposed that they differed materially in the 

 Strophomenidce and primitive Orthidce. In both cases the 

 brachia consisted of low spires of few turns with their axes 

 approximately at right angles to the plane of separation of 

 the valves. In Platystrophia, on the other hand, the strong 

 tendency toward a profound sinus, together with the great 

 extension of the cardinal angles, and general large size and 

 robustness of the shell, indicate that the brachia were well 

 developed and that probably the axes of the spires lay in the 

 transverse diameter of the shell. The striking difference 

 between Platystrophia and all the other Orthidce may very well 

 have been due primarily to this peculiarity. If the modifica- 

 tions of the sinus are correlated with modifications of the 

 brachia, this view is certainly well founded, for the main lines 

 of evolution of this genus are as a matter of fact marked out 

 by modifications affecting the sinus. | If, further, the modifi- 

 cation of the brachia was progressive and affected the entire 

 genus, we have a perfectly adequate reason for the parallel 

 evolution of the lynx and biforata groups, and an explanation 

 of the homoeom orphic forms mentioned above. Thus a wide 

 separation of the two lobes of the brachia would account for 

 the profound sinus and obsolescence of the lateral plications of 

 the sinus in P. costata and P. laticosta unicostata and for the 

 ultimate bilobation of the shell in Bilobites. The force of this 

 will be seen at once if we compare such a form as Spirifer 

 mucronatus (which Platystrophia so closely resembles) in 

 which the sinus is deep, with Meristina in which it is shallow 

 or lacking. In the former the bases of the spires are widely 



-Thecidium, the only survivor of the Strophomenacea, has a rather feeble 

 development of the brachia. Since, however, we are led to believe that this 

 is a degenerate type, no safe conclusions in regard to the morphology of the 

 brachia of ancient Strophomenacea can be based upon it. 



f Davidson, Sil. Brachiopoda, 1869, pi. xxxix, fig. 16. 



% Ibid. pi. xxxvii, figs. 25, 26. 



^' I\ rjirjanteus, Zittel, Textbook of Paleontology (Eastman Ti\), p. 318, fig. 

 532 B. 



|| It may, of course, be argued that external factors caused changes in the 

 depth of the sinus and that these changes caused readjustments of the 

 brachia. This is gratuitous assumption. Thei-e is absolutely no reason why 

 the mantle of a Brachiopod should sag in this particular region except that 

 at this point it lacks the support of any internal structure. The development 

 of a sinus under such conditions is a mechanical necessity. 



