NO. 5 RHYNCHONELLOID BRACHIOPODS — COOPER 37 



thickening of the shell is not great. The degrees of development of 

 the dental plates are, in this case, not regarded as generic in character. 



PROBOLARINA Cooper, new genus 



(Gr. probolos, projection) 



Plate 17, A, B 



Subpentagonal to subtriangular in outline, with the greatest width 

 at or near the middle ; inequivalve, the brachial valve having the 

 greater depth and convexity ; anterior commissure uniplicate ; surface 

 semicostate, the posterior third to half smooth, anterior half to two- 

 thirds costate. Beak moderately long, pointed, nearly straight ; fora- 

 men small, longitudinally elliptical, hypothyrid to submesothyrid and 

 with strongly auriculate margins. Deltidial plates prominent, wholly 

 visible, conjunct throughout their length and anteriorly resting on the 

 umbo of the brachial valve. 



Pedicle valve interior with strong pedicle collar, small teeth sup- 

 ported by vertical dental plates separated from the side wall by narrow 

 umbonal chambers. Details of the musculature not available. 



Brachial valve interior with narrow sockets bounded by erect but 

 not greatly thickened socket ridges; crural bases attached to socket 

 ridge by a prominent, flat outer hinge plate ; crura f alcif er, long, scimi- 

 tarlike, crescentic in section and convex outward. No cardinal proc- 

 ess. Muscle and pallial marks not visible in available material. 



Type species. — Rhynchonella holmesii Dall, Trans. Wagner Free 

 Inst. Sci., vol. 3, pt. 6, p. 1536, pi. 58, figs. 10, 12 (not 11), 1903. 



Comparisons. — This genus is most like Rhytirhynchia in its exterior 

 characters but differs importantly in the interiors as well as in details 

 of the exterior. Rhytirhynchia has almost nude valves except for the 

 costation at the very anterior margin. In Proholarina on the other 

 hand the costation affects more than two-thirds of the valve, only the 

 umbones being free of costation. The deltidial plates of the two 

 genera are conjunct and both are auriculate but those of Proholarina 

 are more developed and more elaborately auriculate than those of 

 Rhytirhynchia. 



Inside the pedicle valve of Proholarina a strong pedicle collar 

 strengthens the beak and strong but thin dental plates buttress the 

 teeth. In Rhytirhynchia on the other hand the dental plates are rudi- 

 mentary in the type species and can be seen only as a trace on the sides 

 of the shell. In R. hataiana Cooper from the Pliocene of Japan mod- 

 erately developed dental plates are present but they are not to be 

 compared with the strong and vertical plates of Proholarina. 



