THE BRACHIOPODA 37 



Among Mesozoic genera the true Rhynchonella (R. loxia) 

 is rare, being only known from the Upper Jurassic of 

 Russia : it is not unlike Pugnax acuminatus in form. Very 

 similar in form, but differing in being smooth instead of 

 finely striate, are the Middle Lias species acuta (now 

 named Homceorhynchia) and the Inferior Oolite cynocephala. 



Somewhat similar, but with more numerous ribs, 

 originating rather suddenly after a long smooth stage, is 

 Tetvavhynchia (R. tetraedra) of the Middle Lias. 



Another very characteristic species-group, not yet 

 given a generic name, is that of " Rhynchonella " concinna 

 of the Bathonian, in which the fold and sinus are very 

 slight, the ribs close-set and rather fine, and the beak 



b 



FIG. g. DELTIDIAL PLATES OF RHYNCHONELLIDS. 



a, Rhynchonella obsoleta, Davidson (non Sowerby), x 2 ; b, R . concinna 

 (}. Sowerby), young, X2; d, deltidial plate. (Original.) 



very acute and prominent. Young specimens of this 

 group are the best on which to study the deltidial 

 plates (Fig. 4, c), which in so many rhynchonellids 

 become obscured by the curvature of the beaks. Acantho- 

 thyris has numerous fine ribs ornamented with spines : it 

 is a Jurassic genus, but a similar spiny rhynchonellid 

 lives off the east coast of Japan. 



The commonest Cretaceous species belong to the 

 genus Cyclothyris : they are rather broad forms, with very 

 fine and close-set ribs, distinct fold and sinus, and delti- 

 dial plates protruding mere or less as a tube around the 

 pedicle. Such are C. latissima of the Lower Greensand, 

 C. grasiana of the Upper Greensand, and C. plicatilis of 

 the Chalk. 



