6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I39 



orthoid ancestor. In the Devonian the cardinal process of some genera, 

 especially the robust forms that have passed under the name Uncinulus 

 (=Sphaerirhynchia), have elaborate cardinal processes. Some of 

 these appear to be secondary characters and difficult to evaluate in the 

 present meager state of our knowledge. The cardinal process is not 

 highly developed in the few Mesozoic forms, the interiors of which 

 have been described. In modern and Tertiary forms the most promi- 

 nent cardinal process is that of Plicirhynchia, a robust and thick shell. 



The cardinal process of several genera such as Notosaria (pi. 6, B, 

 fig. 16) and Hemithyris (pi. 4, E, fig. 9) appears as a triangular 

 roughened area at the apex. In the younger shells it is scarcely visible 

 but it is fairly prominent in old or obese specimens. The majority of 

 the modern and Tertiary forms have no cardinal process, the diductor 

 muscles being inserted in a pit under the apex. The presence of a car- 

 dinal process in rhynchonelloids of this age is thus a ready means of 

 distinction. 



Hinge plates. — These structures are an important part of the car- 

 dinalia and the combination of them with various kinds of crura makes 

 recognizable patterns. The sockets, which are corrugated in nearly all 

 of the genera discussed herein, are defined by a prominent ridge that 

 curves anterolaterally from the apex or the cardinal process to form a 

 narrow cup defining the socket. This ridge may be high or low, thick 

 or thin, and to its inner side is attached the outer hinge plate or the 

 crus, depending on the genus. The outer hinge plate may not exist in 

 some genera or it may be a fairly broad plate between the socket ridge 

 and the crus. To it are attached the muscles that rotate the animal 

 on its pedicle. The outer hinge plates are especially well developed in 

 Basiliola (pi. 12, fig. 15) and Neorhynchia, but not present in 

 Aphelesia. 



The inner hinge plates are seldom well developed but appear in 

 several genera. These are extensions medially from the inside edge 

 of the crura. They are best developed in Frieleia (pi. 15, A, fig. 10) 

 where they are so strong that they unite in the middle of the valve to 

 create a small apical chamber somewhat reminiscent of the septalium 

 (or cruralium?) of certain Paleozoic and Mesozoic genera. The inner 

 hinge plates are also developed in exaggerated form in Aetheia but 

 in a way different from Frieleia. In Aetheia they are not flat or slightly 

 concave plates but are great swellings that extend medially from the 

 crura and plug the whole apical region. The degree of development 

 of either of these hinge plates may play a role in genus definition. 



Crura. — The crura are the most distinctive part of the rhynchonel- 



