MORPHOLOGY OF THE ORTHOID SHELL 19 



COMMISSURES 



When viewed from the front or anterior, the line of contact of the two valves is primitively 

 straight but in most derived genera it is more or less undulated. The same holds for the lateral com- 

 missure. Buckman has analyzed the flexures of the anterior commissure in Mesozoic rhynchonellids 

 and terebratulids, and has named nine stages developed out of the primitive straight or recti marginate 

 one. Among the orthids only two of the Buckman stages can be recognized, namely the sulcate and 

 uniplicate ones; both of these are the simplest of the modifications of the rectimarginate stage. In 

 the sulcate stage there is a single sulcus in the dorsal valve and a fold in the ventral, as seen m Aula- 

 cella, Enteletim, or Parenteletes. This type of commissure is rather uncommon. In the uniplicate 

 stage, these conditions are reversed and there is a single fold on the dorsal valve and a sulcus m the 

 ventral one. This is the more common condition and is well exhibited by Platystrofhia. 



The conclusion that a fold and sulcus, or what amounts to trilobation, may have been developed 

 to facilitate the passage of the incurrent and excurrent streams of water used in the aeration of the 

 mantle and for food-gathering was advanced by Orton. 



The tendency to develop a fold and sulcus is inherent in most Orthoidea but in some of them, 

 for example Hebertella, there is little stability as to which valve shall receive the sulcus. In some 

 stocks there is a marked reversion of the fold and sulcus, as in Eridorthis and Thiemella. 



The common condition of the lateral commissure is a more or less strong flexmg toward the 

 dorsal valve. This is true of Valcourea and many other forms. The primitive condition is an 

 unflexed lateral commissure. A ventrally flexed lateral commissure is rare. 



CONVEXITY 



In the Cambrian, most shells are normally biconvex, as is the first shell growth in all brachi- 

 opods, and here it is also usual to find the ventral shell the more convex and the deeper. In later 

 forms the reverse condition is, however, of common occurrence and is independently originated over 



and over again. . . ,. , . ,• • 4.1- * *u» 



The great majority of brachiopods, viewed in longitudinal section, are biconvex, in that the 

 external curvature of both valves is convex, with the ventral the more so and therefore deeper than 

 the dorsal This common condition is called the lenticular or biconvex phase hy Buckman. More 

 rarely it is the dorsal shell that is more convex, and such forms can be described as dorsi-biconvex. 



Resupination is a condition that has been reported commonly in the orthids, but unfortunately 

 the term has never been defined with any precision. Some authors regard brachiopod shells as 

 resupinate when the ventral valve has less volume than the dorsal. Atryfa wou d be a fine example 

 of such "resupination," but this is not what the term apparently meant originally. As applied by 

 Hall and Clarke, it embraces such shells as have the ventral valve concave and the dorsal convex. 

 According to this view, Valcourea and Strofhomena would be two unrelated examples of resupina- 

 tion However, there is a marked difference between Valcourea (and all other convexo-concave 

 orthids) and Strofhomena in the manner of the reversion of convexity in the ventral valve. In the 

 young stages of Strofhomena the ventral shell is convex and may carry a low fold; the dorsal valve 

 is essentially flat and may be gently sulcate. In later stages, however, the dorsal shell becomes 

 strongly ventricose and the ventral one deeply concave. In the convexo-concave stage of the 

 orthids, on the other hand, as in Valcourea and Hebertella, the dorsal valve never has the incipient 

 flat stage as seen in Strofhomena, and this affords an easy external way of distinguishing a stropho- 

 menid from an orthid homoeomorph. Consequently in this work we are restricting the term resufma- 

 tion to the condition seen in Strofhomena and its allies, while the other condition among orthids 

 may be called fseudoresupination. 



^ Orton. J. H., On Ciliary Mechanisms in Brachiopods and some Polycha:tes. Jour. Marine Biol. Assoc., U. K., 

 new ser., vol. 10, 1914, pp. 283-311. 



