GIRTY, NEW CARBONIFEROUS FOSSILS 217 



fested at an early stage. The outline usually contracts toward the hinge, 

 and the ears are small and inconspicuous. 



Ventral valve strongly convex with gradually enlarging umbo. Of course, 

 in the narrow specimens the umbonal angle is more acute than in the broader 

 ones. Ears small and depressed. There is usually a broad, shallow, some- 

 times indistinct median sinus. 



In the dorsal valve the shell is gently concave over the visceral area, more 

 strongly flexed around its border. A median fold is usually present. The 

 ears are small and indistinct. 



The costfe vary greatly in character. They are usually rather irregular, 

 with relatively broad strife in between. At rather frequent and regularly 

 increasing intervals they give off small spines and are swollen and elevated 

 at the spines and constricted and depressed just in front, so that in some cases 

 the surface looks as if marked less by continuous costse than by elongated 

 spine bases which terminate rather abruptly at the anterior end with the 

 development of the spine which gave rise to them. This effect is more marked 

 in some specimens than in others, and also in some specimens the costre are 

 finer and more closely arranged than in others. Toward the front, the costse 

 tend to be more regular and continuous. Distinct strife of growth usually 

 show upon well-preserved specimens, to which are in some cases added trans- 

 verse wrinkles more or less irregular and obscure, except on the ears. In 

 some cases, also, there are well defined, regularly arranged transverse bands. 

 The arrangement of the spines is more regular in some specimens than in 

 others, and occasionally they appear to occur in transverse rows, especially in 

 connection with the sub-lamellose bands just mentioned. 



In the dorsal valve, the sculpture is the reverse of that described. In the 

 most strongly characteristic specimens, the external mold appears to be marked 

 by sharply defined regular spine bases with prominent spines. In others, 

 the appearance is more that of continuous costse. Regularly concentric sub- 

 lamellose bands frequently occur, and spines are developed on this valve, as 

 well as on the other. 



Productus arkansanus Aar. multiliratus var. nov. 



One or two localities have furnished a phase of this species which 

 seems to warrant discrimination as a distinct variety. It is characterized 

 by being unusually large, broad and with very fine continuous lirge and 

 small spines. The dominating form at these two stations, it is yet even 

 there associated with examples which can most appropriately be referred 

 to the original species, while with the latter an occasional specimen is 

 found which, by reason of its finer markings, might perhaps be referred 

 to the variety multiliratus. Because of this intergradation, more or less 

 complete, the present form could hardly be considered more than a 

 variety. 



Diaphragmus subgen. nov. 



This name is introduced for Productus elegans Norwood and Pratten, 

 a specific name for which Worthen later substituted cestriensis. The 



