36 NOTES ON THE BRITISH JURASSIC BRACHIOPODA. 



species and six varieties are recorded for the other divisions 

 of the Lower and Middle Oolites. R. suhvariabilis and 

 inconstans from the Kim. clay, and R. portlandica from the 

 Portland rocks are the only three species found in the 

 Upper Oolites. 



The Rhynchonellse are undoubtedly the most difficult and 

 troublesome group of brachiopods which the student has to 

 contend with, that is so far as the species are concerned. 

 In the same species we often find specimens very convex 

 and gibbous, while others are comparatively depressed or 

 flat ; the number of plaits or ribs in the valves of the same 

 species may vary considerably, these not infrequently form 

 a more or less well pronounced or elevated mesial fold, 

 which in the same species may consist of only one plait, or 

 of two, or three, as in R. cynocephala. 



The shape and position of the beak and the deltidial plates 

 also vary. 



Another great difficulty we sometimes experience is the 

 great difference of shape assumed during the different stages 

 of growth. The shell in the young state may be depressed, 

 with beak straight and deltidial plates clearly shown, but 

 in the adult the shell may be very convex, the beak quite 

 incurved and the deltidium quite obscured, as is sometimes 

 the case in R. varians^ var. smithii, concinna, etc. 



Then again in the young we may have the plaits well 

 shown, extending right down from the umbo to the front 

 margin of the shell, while in the adult these may become 

 evanescent towards the umbo, as in some forms of R. lineata, 

 var, radstockensis ; or the simple plaits of the young form 

 may unite in pairs in the older stage so as to become bifur- 

 cated, as in R. rimosa, furcillata, etc. 



It is therefore necessary to take all these general char- 

 acters into consideration when arranging the species of 



