NO. 5 RHYNCHONELLOID BRACHIOPODS — COOPER 5 



are extended ventrally to the valve floor where they join to form the 

 collar, but this is never closed at the anterior end. In genera with dis- 

 junct deltidial plates the collar is seldom prominently developed. In 

 some instances as in Frieleia it forms a callosity at the posterior apex 

 against which the pedicle rests. It is suggestive of the pedicle callist of 

 the Orthoidea in the Paleozoic. In Cryptopora no pedicle collar is de- 

 veloped, but a small apical plate elevated above the valve floor serves 

 the same purpose, the pedicle evidently lying against it. 



The dental plates are another part of the apical region of the rhyn- 

 chonelloid of importance in classification and generic definition. Den- 

 tal plates are generally present in rhynchonelloids from the time of 

 their origin. They are present in all but two of the Recent and Tertiary 

 members discussed herein. Usually they are strong and erect plates 

 which define narrow but distinct umbonal cavities. In a few genera 

 such as Rhytirhynchia, Aetheia, and Patagorhynchia the dental plates 

 are reduced to mere vestiges or are absent. The only specimen of 

 Patagorhynchia available for dissection, that figured on plate 6, A, 

 failed to show any trace of dental plates. Aetheia which is usually 

 described as lacking these structures seems to have vestiges of them. 

 Rhytirhynchia has fairly distinct dental plates in the Okinawa Pliocene 

 species but they are mere vestiges in the Recent R. sladcni (Dall) 

 from the Indian Ocean. 



INTERIOR CHARACTERS OF THE BRACHIAL VALVE 



The definitive family characters of the brachiopods are in the 

 brachial valve. This is the more conservative of the two valves and 

 thus retains its diagnostic features while parts of the pedicle valve 

 which is fixed to some solid object may be evolving. The most im- 

 portant characters of this valve are the cardinalia which embrace the 

 cardinal process, the hinge plates, crura, and septa. Except for 

 Thomson's (1927) work, no attempt has been made to apply the fea- 

 tures of the cardinalia to the classification of Recent and Tertiary 

 rhynchonelloids. Parts of the cardinalia have been used in defining 

 families and subfamilies of the Paleozoic rhynchonelloid genera. 

 These attempts have been based on the presence or absence of a car- 

 dinal process. The type of crura and hinge plates, however, have not 

 been used even though they offer the greatest possibilities. 



Cardinal process. — In the modern and Tertiary brachiopods this 

 structure does not attain a high state of development and makes little 

 impress on the classification. In some Paleozoic genera the cardinal 

 process is a simple vertical blade, suggesting inheritance from an 



