NO. 5 RHYNCHONELLOID BRACHIOPODS — COOPER 9 



having broad-bladed, gently curved crura that are convex outward and 

 gently concave inward. This crus is generally attached to the hinge 

 plate by its convex side and may or may not be separated from the 

 socket ridge by outer hinge plates. 



The third type of named crus is that characterizing the Erym- 

 nariidae and called septifer type. This is an extremely rare type of 

 crura known in a few genera only. 



A fourth type of crus is recognized in the Cryptoporidae. The 

 crura are long and slender and appear to be continuous with the distal 

 end of the socket ridge. The distal extremity of the crus is com- 

 monly flattened, expanded, and serrate or digitate, some examples 

 suggesting a tiny hand with outspread fingers. The name "manicn- 

 lifer" is proposed for this type. 



The fifth type of crus in the modern and Tertiary genera is gen- 

 erally shorter than the others, laterally compressed, somewhat flat in 

 section and attached to the hinge plate or socket ridge so that the 

 short direction is nearly vertical or slightly oblique. This type is best 

 seen in Frieleia, but Grammetaria, Hispanirhynchia, and Compso- 

 thyris have similar crura. This type is here designated as "spinulijer." 



Median ridges, median septa and camerae. — A conspicuous feature 

 of many rhynchonelloid stocks is the median septum. Some groups 

 however, such as the Basiliolinae, are devoid of septa in the brachial 

 valve. The most conspicuous septum in any modern rhynchonelloid 

 is that of Cryptopora in which, although short, it is so elevated that 

 it almost touches the inner surface of the opposite valve. The septum 

 of Frieleia is also considerably elevated. 



In the descriptions below a distinction is made between median 

 septa and median ridges. The term septum is reserved for the thin 

 blades, like those of Cryptopora or Frieleia that stand boldly and 

 abruptly above the inner valve surface. These are in contrast to the 

 ridges such as that of Aetheia, which is low, short, and stout, and that 

 of Aphelesia which is low, long, and slender. Dorsally aseptate rhyn- 

 chonelloids are commonly provided with low and inconspicuous median 

 ridges, some in the form of a myophragm but others buttressing the 

 cardinalia. 



In a few genera of Recent and Tertiary, Frieleia for example, the 

 median septum joins folds from the inside of the crural bases to form 

 a small chamber at the posterior. In some instances the chamber re- 

 mains open but it is frequently closed by deposit of shell material on 

 its inner walls to form a thick apical callosity. All degrees occur in 

 Frieleia from the open chamber to the solid callosity between the 



