SILURIAN BRACHIOPODA 65 



SPECIFIC CHARACTERS 



Mature Form (plate 5, figs. 19, iga, i()b'). Shell small, suboval or 



cordate, often subpentagonal ; plano-convex, greatest'width along the hinge. 



Ventral valve convex, depressed at the sides ;^beak exsert, and in old 

 forms arcuate. 



Dorsal valve flat and sometimes depressed near the beak. The 

 ventral valve is marked by two very prominent plications which pass 

 along the deep median sinus, and are accompanied by two less distinct pli- 

 cations on each of the latera, making in all six plications, of which the 

 two nearest the hinge-line are sometimes obsolescent. The dorsal valve 

 bears a prominent median fold, and two well-marked plications on the lat- 

 eral portions of the shell. Toward the margin are concentric laminae of 

 growth. An average adult individual measures 5.5 mm. in length by 5 mm. 

 in width. 



Variations in Outline. The individuals divide themselves into 



two groups according to their outline : 



(a) normal form. 

 (<5) long form. 



The first of these groups includes the great majority of all individuals 

 found, which are characterized by a relatively broad figure and subcircular out- 

 line. Members of the second group are comparatively few in number, and 

 are elongate or subovate individuals. The long form (p) is well defined in 

 immature growth-stages, and appears to be a permanent varietal difference. 



Abnormalities. A variation in adult shells, noticed only in rare 



instances, is a tendency to an asymmetrical development in the plications, as 

 shown on plate 5, figure 18, where, by unequal growth upon the lateral por- 

 tions of the shell, the median plication on the dorsal valve is deflected to one 

 side, and the corresponding median sinus on the ventral valve displaced, 

 the axial line of the shell being occupied by one of the strong plications 

 bounding the sinus. Another form of this asymmetry is manifested in the 

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