FROM NORTHWESTERN EUROPE 43 
graphical range of the British Rhynchonellas s.s. is also indicated. Some of the 
forms figured by Makridin from the Russian Platform seem to be quite close to 
British specimens although the variation of the genus was such that none of the 
forms from these two areas is considered to be conspecific. 
The most distinctive British element seems to be the form with a long, posterior 
smooth stage and strong, short ribs anteriorly. This form is represented in the 
specimens collected by Prof. M. R. House, mentioned above, and is the basis on 
which such species as R. vivelensis and R. triplicosa are included in the genus. These 
species are all characterized by the above features and seem to comprise a persistent 
stock which is first seen in the basal Callovian and continues to the Volgian. It 
appears that the form represented by loxiae and portlandica is restricted to the 
Volgian. 
Rhynchonella rivelensis Loriol 
(Pl. 5, figs. 3-6, text-fig. 18) 
1904 Rhynchonella rivelensis Loriol: 282-83, pl. 26, fig. 43. 
EMENDED DIAGNOSIS. Small to medium sized Rhynchonella ; subpentagonal in 
outline ; slightly incurved interarea ; large smooth area posteriorly about Io strong, 
subrounded ribs anteriorly ; crura radulifer. 
STRATIGRAPHICAL RANGE. Oxfordian—upper transversarium and lower pseudo- 
cordata zones. 
TYPE SPECIMEN. Lectotype, here selected, fig. 43, pl. 27 of Loriol (1904), the 
specimen came from the “‘ Argovien II’ of Mont Rivel, France. 
MATERIAL. 60 specimens from Alex near Oyonnax (Ain) ; 2 specimens from 
Choux du Dombief (Jura). All the material was collected by Dr. Enay and sub- 
sequently donated to the author. 
DESCRIPTION. External characters. R. rivelensis is subpentagonal in outline and 
is either equally biconvex or with the brachial valve the more inflated. There is a 
low uniplication in the anterior commissure and a low fold on the brachial valve. A 
linguiform extension is only poorly developed. The coarse, simple ribs are only 
developed on the anterior half to one third of the shell ; concentric ornament 
consists of growth lamellae, most clearly seen on the posterior smooth area, and very 
fine growth lines. 
The beak is small, suberect and sharply pointed. Beak ridges are present and 
limit the incurved interarea. The deltidial plates project slightly around the hypo- 
thyridid pedicle opening. 
DIMENSIONS OF FIGURED SPECIMENS. 
length thickness width 
I-16 cm 0:66 cm 1:22 cm 
E07 ci 0-77 cm 1-25 cm 
Internal characters. Pedicle valve. The quadrate delthyrial cavity is bounded by 
long subparallel dental lamellae ; the lateral cavities are well developed. The 
teeth are strong but lateral denticulae are scarcely differentiated. 
