1 2C) Qu in mgs — Morphogenesis of Platystrojphia. 



The observations which follow assume that the history of 

 Ptctiystrophid) outlined above, is in the main correct; and 

 have suggested themselves from time to time as the investiga- 

 tion proceeded. 



Va)'i((hi!it»j. — Platystrophia presents its highest degree of 

 variability near the beginning (epacme) of its history, and 

 thenceforward becomes progressively less variable, till finally 

 only characters of no physiological importance are affected. 

 In other words, variability is progressively restricted to char- 

 acters of lower and lower taxonomic value* 



Considering the entire group, the presence of a ventral sinus 

 is a generic character from the early Ordovician on ; but the 

 ancestral form, Orthis lenlicularis, has the sinus in the dorsal 

 valve (as in the nepionic shell of Platystrophia), so that at 

 some time between the Cambrian and Trenton the type varied 

 in this respect. Since, moreover, the fold and sinus of Brachi- 

 opods are undoubtedly due to the presence and position of the 

 brachia, such a transfer of the sinus from one valve to the 

 other indicates a considerable readjustment of the internal 

 organs, and. of the brachia in particular. 



In all forms of Platystrojphia above the basal Trenton, the 

 sinus is either definitely biplicate or definitely triplicate ; and 

 this character delimits the primitive species biforata and lynx. 

 In the early Trenton, however, we meet with both biplicate and 

 triplicate shells associated together, and the inference is that at 

 that time the mode of origin of the plications of the fold and 

 sinus was variable. In the lower Lorraine the pauciplicate P. 

 lynx has indifferently one, two, or three plications in the sinus ; 

 but a little later these peculiarities tend to become fixed in distinct 

 types, so that ultimately laticosta, and especially its derivative 

 acutilirata,h&ve almost constantly three plications in the sinus, 

 while Bilobites derived from costata has the single median 

 plication of the latter fixed as a persistent nepionic character 

 which is absolutely invariable. Throughout the Trenton and 

 Lorraine P. lynx varies extensively in contour and number of 

 plications, the limits being between six and twelve plications 

 on each side of the fold and sinus, and the average seven on 

 each side. In the late Richmond, where lynx reappears in 

 great numbers, it has almost constantly between nine and 

 twelve plications on each side of the fold and sinus, and the 

 shell index is uniformly high. The sinus index, which seems 

 to have no fixity at all in Lorraine forms, is in acutilirata 

 fairly constant. The early forms of the P. biforata group, 

 according to European authors, vary extensively in contour 

 and number of plications, and this variability is still high, 

 especially the number of plications, in our Clinton forms; but 

 * Cf. Hyatt, Proc. Bos. Soc. Nat. Hist., xxxii, 1895, p. 371. 



