Oumings — Morphogenesis of Platystrophia. 7 



Orthis s. s. Dinorthis, Pleetorthis, etc.; and on the other the 

 multiplicate group Hebertella, Dalmanella, etc. 



In the Middle Cambrian, Orthis Hicksii Salter,* fig. 2b, has 

 many of the nepionic characters of Platystrophia and may be 

 the ancestor of Orthis lenlicularis. 



The Lower Cambrian furnishes very few Brachiopods of an 

 Orthid aspect : but 0. salemensis Walcott,f an outline figure 

 of which is given here (fig. 2c), again strongly suggests the 

 nepionic shell of Platystrophia. In the present state of our 

 knowledge of Cambrian forms it is perhaps unsafe to specu- 

 late far in regard to phylogenies of any sort ; nevertheless I 

 am inclined to believe that some such form as 0. salemensis 

 constitutes the final link between the Orthidce and the primi- 

 tive -Brachiopoda of the type of Kutorgina cingulata. 



II. Neanic stages. — The specific characters of Platystrophia 

 do not begin to appear until the shell has reached a breadth 

 of 3 mm or more. At about this size, the demarcation of the 

 true fold and sinus, which began at a little over 2 mm , has pro- 

 duced a noticeable sinuosity of the margin (as viewed from the 

 front). At the bottom of the sinus is a single plication — the 

 same that formed the ventral fold of the nepionic shell. On 

 either side of the sinus are four or five plications (including 

 the ones immediately bounding it). The dorsal valve has the 

 two median plications slightly elevated, forming a fold ; and 

 the sulcus between them represents the continuation of the 

 median dorsal sinus of the nepionic shell. (These stages are 

 shown in fig. 3, TV, Y.) 



A little later (fig. 3, YI), usually at about the breadth of 

 4 mm , two additional plications make their appearance in the 

 sinus at its front margin, one on either side of the primary 

 plication. In the dorsal valve at this stage the two plications 

 of the fold are each seen to bifurcate, giving four in all. The 

 typical number of plications of the fold and sinus of P. lynx 

 are now present, though the lateral slopes still possess only 

 five or six. Further increase in the number of plications of 

 the slopes takes place by addition at the cardinal angles, and 

 never (with the rarest exceptions) by bifurcation of those 

 already formed, or by implantation between them. In a shell 

 of 6 or T mm breadth it is usually possible to determine the 

 variety, therefore the neanic period may be said to cease at 

 that size. 



In well preserved material it is possible to study the neanic 

 stages at the beaks of adult specimens ; and these stages and 

 those immediately following (early ephebic) are of the utmost 

 importance in tracing lines of descent within the genus. Three 



* Davidson, Silurian Brachiopoda, 1869, p. 230. 



f Tenth Ann. Rep., U. S. G. S., 1891, p. 612, pi. 72, fig. 6. 



