NO. 2297. BRACHIOPOD GENUS PLATYSTROPHIA—McEWAN. 389 



fold are disregarded, the neanic PlatystropMa is almost a Plectorthis. 

 There is httle doubt that when the nepionic shell of Plectorthis is dis- 

 covered it will be found to be quite indistinguishable from the nepionic 

 shell of PlatystropMa, since the two priesent at the beaks almost iden- 

 tical characters. * * * Plectorthis may therefore represent an 

 offshoot from the Platystrophia group near its initiation or, as sug- 

 gested above, it may have been separately derived from the Orthis 

 lentic'ularis stock." 



Beecher has shown that Platystrophia has all the characters neces- 

 sary for the ancestor of Bilohites} Professor Cumings carried this 

 investigation somewhat further and found that the nepionic stage of 

 Bilohites is almost identical with that of Platystrophia crassa in every- 

 thing except size. He says: "The median plication of the sinus of 

 unipHcate and triplicate types of Platystrophia is a character that is 

 never absent, whatever other modifications may affect the shell; the 

 presence of this persistent character in a derived genus is to be 

 expected, and affords, together with the evidence from development, 

 a virtual demonstration of the derivation of Bilohites from Platy- 

 strophia."^ The ancestor of Bilohites must have been uniplicate and 

 he regards Platystrophia crassa as the most likely ancestor. 



As speciahzed forms of this species are taking on declining charac- 

 ters, it is likely that Bilohites originated from one of the least special- 

 ized members. 



GROUPS AND SUBGROUPS OF THE GENUS. 



On the basis of certain neanic characteristics the species of Platy- 

 strophia were found to be separable into three groups wliich Professor 

 Cumings has called the Uniplicate, Biplicate, and Triplicate types.^ 



1. Uniplicate group. — In the uniplicate group the one phcation in 

 the sinus and two on the fold, at the end of the nepionic stage of 

 development, continue unmodified throughout the entire life history, 

 and there is no further modification of the fold and sinus except that 

 both these parts and the phcations which they bear become broader 

 as they approach the frontal margin. 



2. Biplicate group. — In the biplicate group the median plication 

 of the sinus bifurcates while on the fold a plication is intercalated 

 in a median position. Some members of this group do not show a 

 further development and are regarded as a subgroup which the 

 writer will later refer to as subgroup A. 



In subgroup B the bifurcation of the plication in the sinus is soon 

 followed by the intercalation of a plication in a median position 

 while on the fold the median secondary plication bifurcates. 



In subgroup C the bifurcation of the primary plication in the sinus 

 is followed by the implantation of a phcation on each of the lateral 

 slopes while on the fold, the two primary plications bifurcate. 



» Amer. Joum. Sci., ser. 4, vol. 42, 1895, pp. 51-56. » Idem., vol. 15, 1903, p. 40. » Idem, p. 10. 



