NO. 5 RHYNCHONELLOID BRACHIOPODS — COOPER 3 



ing genus described and illustrated herein. Dr. L. G. Hertlein, Uni- 

 versity of California at Los Angeles, lent paratypes of Eohemithyris. 

 Dr. E. Montanaro Gallitelli, University of Modena, Italy, presented 

 the National Museum with specimens of Rhynchonella polymorpha 

 (Massalongo) that made it possible to prepare the interior details of 

 the cardinalia described and figured in this monograph. 



RHYNCHONELLOID MORPHOLOGY 



Throughout geological time the rhynchonelloids have been charac- 

 terized by triangular to subpentagonal form, with prominent beak and 

 a strong median fold on one valve, usually the brachial valve, and a 

 deep sulcus on the other. Nearly all the genera are provided with a 

 conspicuous beak having a foramen modified by deltidial plates. In 

 some genera, especially a few of the Recent, Tertiary, or Mesozoic 

 ones, the deltidial plates are elaborately auriculate, a feature unusual 

 in other brachiopods. The rhynchonelloid shell is commonly costate ; 

 smooth forms are usual in Recent, Tertiary, and Mesozoic families 

 but rare in Paleozoic representatives. The distinctive feature of the 

 rhynchonelloid interior is the more or less long curved crura and 

 hinge plates which characterize the cardinalia. Details of these latter 

 features have long been neglected. 



BEAK CHARACTERS 



It is not here the intention to discuss these characters for all the 

 rhynchonelloid brachiopods but to point out the significant features 

 shown by the genera discussed herein. Of modern and Tertiary 

 rhynchonelloids only Cryptopora and Mannia do not have a small 

 round or elongate-oval foramen. In the two genera mentioned the 

 foramen is elongate-triangular and is restricted only slightly by at- 

 tenuated deltidial plates, which, unlike most other modern and Tertiary 

 genera, form an elevated rim on the sides of the delthyrium. 



The nature and completeness of the deltidial plates usually define 

 the form of the foramen. In some genera the deltidial plates are dis- 

 junct, that is, they do not meet on the anterior side of the foramen. 

 In such cases the foramen is said to be incomplete. An excellent ex- 

 ample of this type is Hemithyris. When the deltidial plates meet on 

 the anterior side of the foramen, they are spoken of as conjunct and 

 the foramen is completely enclosed. Examples are Basiliola, Aphelesia, 

 and Aetheia. These two conditions of the deltidial plates, disjunct or 

 conjunct, are given considerable weight in genus making by some 



