NO. 5 RHYNCHONELLOID BRACHIOPODS — COOPER II 



of all of them. Consequently he placed many species together which 

 are utterly unlike internally. 



In modern and Tertiary rhynchonelloids a smooth exterior is com- 

 mon, a capillate or costellate exterior is also fairly common, but a 

 strongly plicate exterior is unusual, occurring in only a few genera. 

 Rhytirhynchia is similar to Basiliola but differs exteriorly by its an- 

 terior oostation. Notosaria and Tegulorhynchia are the only com- 

 pletely strongly costate modern rhynchonelloids. Rhynchonclla grayi 

 Woodward, the true generic affinities of which are with the Eocene 

 genus Eohemithyris, is partially costate but not so strongly costate as 

 the Patagonian genus Plicirhynchia. 



Folding. — The anterior commissure of most rhynchonelloids is 

 uniplicate but some of them are sulcate or even more complicated. The 

 production of a fold is thought to be related to the feeding habits of 

 the brachiopod, the median fold helping to channel the excurrent 

 stream at midvalve. 



Sulcation, brachial valve with sulcus, pedicle valve with fold, is not 

 a common feature of the brachiopods but crops up again and again 

 in many unrelated stocks, producing confusing heterochronous and 

 isochronus homeomorphs. Neorhynchia is the only known Recent 

 sulcate rhynchonelloid, but another modern deep-sea form, Abys- 

 sothyris, is a terebratuloid having a shape identical to that of Neorhyn- 

 chia. If it were not for the punctae of Abyssothyris it would be almost 

 impossible to tell the two genera apart on exterior characters alone. 

 Rhynchonelloids of almost identical form to Neorhynchia are known 

 from all the periods of the Mesozoic era and from the Paleozoic back 

 at least as far as the Devonian. It is difficult to suggest any reason 

 for the reversal of folding from the usual uniplicate condition because 

 the two types must have functioned in the same manner. It is a com- 

 mon feature of the young brachiopod to have a more robust pedicle 

 valve more or less prominently folded in the ventral direction and with 

 a somewhat sulcate brachial valve. Perhaps sulcation is merely a re- 

 tention of youthful shell characters into the adult stages. 



Several of the modern and Tertiary brachiopods have rectimarginate 

 anterior commissures. This, too, is a youthful character. Buckman 

 emphasized the folding of brachiopods and used this feature as a 

 major part of his classification. It is evident, however, from the above 

 remarks and known brachiopod history that folding is of value only as 

 a generic character. When combined with ornamentation features as 

 Buckman advocated, valid genera have been established. These how- 

 ever can only be placed in their proper families by determination of 

 their beak and cardinalia characters. 



