236 PROC. COTTESWOLD CLUB vol. xili. (4) 



From form attained in IX. b. various lines of development proceed. 

 One begins uniplication directly, which subsequently passes into biplica- 

 tion, and possibly quadriplication. Another elongates the valves before 

 plication commences ; another develops multiplication directly. 



X a. Dorsal plica developed. Tereb. Etheridgii ; T. Wrighti; 



(compare \bb.') ^\ T. ampla. 



b. Subsequent sulcation of T. globata series, and a large 

 plica produces biplica- number of the biplicate Tere- 

 tion. y\/^ brat idee. 



The formation of the sulcus is perhaps a case of hypostrophy to 

 the form of I. a.b. 



T. ivithingtotiensis illustrates a. and b. ; perhaps Plesiothyris does 

 too. T. niaxillata illustrates a tendency towards quadriplication. 



A slightly different course is taken by some other species. 



XA a. Dorsal plication and al- T. sliirburniensis ; T. hyalina, 



most contemporaneous sp. n. 

 flattening of the plica, n 

 b. The flattening proceeds T. Eudesi. 



to sulcation, and so bipli- 

 cation is produced. ^ 



A large number of the biplicate Jurassic Tcrebratuhe would be compre- 

 hended in Divisions X, XA. There is, probably, no real difference 

 in method except that perhaps in X the uniplica is persistent awhile 

 before any infolding begins, while in XA a certain infolding (flatten- 

 ing) begins early and has some duration before definite sulcation 

 commences. 



Many of the other biplicate Terebratula followed the course of first 

 elongating the valves : thus from IX^. there is : — 



XI a. Elongation of valves be- T. punctata ; T. Buckmani, etc. 



fore plication. 



b. Dorsal valve with uni- T. cortoncnsis \ T. Bucki!iani,m 

 plica. f\ some cases. 



c. Sulcation of plica pro- T. crickleyensis ; T. Phillipsi ; 

 duces biplication. /\/\ T. Phillipsiaua. 



