Cuming 8 — Morphogenesis of Platystrophia. 129 



omorph of P. lynx of the upper Lorraine of this country : but 

 it is specifically distinct. Again in the upper Richmond of 

 this country is a form of P. lynx similar in almost every re- 

 spect to P. biforata of the upper Ordovician and Clinton ; yet 

 again specifically distinct. The parallelism of the two series 

 lynx and biforata is complete. Examples of homceomorphy 

 besides those just mentioned, are P. dentata and P. lynx pau- 

 ciplicata, biplicate form, — almost precisely alike in the adult, 

 but differing in the early stages (neanic) : P. costata and P. 

 laticosta unicostata, — alike in number of plications and pli- 

 cation of the fold and sinus but differing slightly in cardinal 

 angle, depth of sinus, and shell index, and very differently 

 derived : the gerontic P. aciitilirata and the upper Rich- 

 mond P. ly?ix, differing only in early ephebic stages. 



The relations of these homoeomorphic forms are indeed sug- 

 gestive. The feature that infallibly distinguishes the P. lynx 

 and P. biforata groups is the mode of origin of the pli- 

 cations of the sinus and fold. Aside from this one character, 

 however, the evolution of the two groups seems to have been 

 determined by the same set of intrinsic characters which re- 

 sponded to the two sets of environmental conditions by similar 

 reactions. These intrinsic characters centered in the progres- 

 sively increasing development of the brachia. It was stated 

 above that the shifting of the sinus from the dorsal valve (m 

 Orthis lenticularis) to the ventral valve (in Platystrophia) was 

 due to the increasing development of the brachia. The grounds 

 for this statement are, the plan of symmetry of the shell which 

 is undoubtedly primarily determined by the brachia, and the 

 pretty general restriction of the sinus to the ventral valve. 

 In those brachiopods (Spiriferacea, Terebratulacea*) in which 

 the extent of the brachia is definitely indicated by their cal- 

 cified supports, the connection between the brachia and the 

 sinus is obvious enough, and the development of the sinus is 

 usually directly related to the extent and position of the bra- 

 chia. In inarticulate brachiopods (Atremata, Neotremata) 

 where there is more freedom of motion of the valves upon 

 each other, a fold and sinus are seldom developed, for obvious 

 reasons. 



As there are no representatives of the Orthidm extant, the 

 condition of the brachia in that group can only be inferred 



* In the Terebratulacea the sinus is more often in the dorsal valve than is 

 the case in any other group. A glance at the brachia of such a shell will 

 show that this is in reality the exception that proves the rale, for here the 

 median lobe of the brachia extends toward the ventral mantle so that the real 

 hiatus between the right and left lobes of the brachia comes on the side 

 toward the brachial valve. This is beautifully shown in the figure of Tere- 

 bratulina coreanica (fig. 13, plate 46) in Morse's recent splendid memoir on 

 living brachiopods (Mem. Bos. Soc. Nat. Hist., v, 1902). Compare with 

 Spirifer striatus, fig. 5?6 A, Zittel's Text book of Paleontology (Eastman 

 Tr.), p. 336. 



