NOTES ON THE BRITISH JURASSIC BRACHIOPODA. 35 



The apophjsary system in the dorsal valve is composed 

 of two short, flattened, and grooved lamellse, separate, and 

 moderately curved upwards, attached to the inner side of 

 the beak of the smaller valve, and to which are affixed the 

 free spiral fleshy labial appendages ; a small central longi- 

 tudinal septum, more or less elevated, extends along the 

 bottom of the smaller valve from under the beak to about 

 half or two- thirds the length of the shell, separating the 

 muscular impressions visible on either side (7, p. 65). 



The genus has a wide geological range, being represented 

 in all the formations, from the Silurian up to the Tertiaries. 

 It is represented in existing seas by its congener Hemithyris, 

 of w^hich five species are known, 



Ehynchonella attained its maximum development in the 

 Jurassic epoch, where it is represented by sixty species and 

 ten varieties.* In this number are included R. hampenensis, 

 S.S.B. (2), from the Inferior Oolite of the Cottes wolds, 

 which has been named since 1884; also B. (?) coronata and 

 lopensis, Moore, which are doubtfully placed in this genus : 

 but not the spinose Rhynchonellse (AcantJiothyris) . 



Of this number no less than twenty-five species and one 

 variety f are found in the Inferior Oolite, while only nine 



* Vide Appendix. 



t Certain of the varieties of this and other genera are considered by 

 some palseontologists to be distinct species ; there are others however 

 who would considerably reduce the number of species and make them 

 varieties only. This difference of opinion, unfortunately, is likely to 

 exist for some time to come. I have therefore recorded as varieties 

 those which Davidson considered to be such, although personally I feel 

 confident that many of them are specifically distinct. For instance, 

 Davidson considered R. suhangulina to be a variety of R. angulina, which 

 in my opinion is a mistake. R. subanguHna occurs much earlier in 

 geological time, viz. in the Murchisoncs zone, whilst R. angulina occurs 

 in the Parkinsoni zone. On this supposition, the variety would have 

 come into existence before the typical form from which it was derived. 



