DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 331 



primary interest. Both the dorsal callosity and pedicle-tube 

 continue to increase in size with the growth of the shell, 

 from the incipient form to maturity. 



Specific Characters. 



Mature Form (Plate XVII, figures 2, 2a, 8). — Shell semi- 

 elliptical, wider than long, the greatest length being along 

 the hinge. The body cavity is very shallow, and the shell 

 has a concavo-convex form. 



Dorsal valve flat in the upper part, moderately convex in 

 front. Ventral valve slightly convex on the umbo, and con- 

 cave over the remainder of the valve. Hinge-area formed by 

 both valves. Ventral area the wider, carrying in the centre 

 a small conical pedicle-sheath which is usually minutely 

 perforate at the apex. Dorsal area linear, with a callosity in 

 the middle, under the pedicle -tube of the opposite valve. 



Test thin, surface ornamented by about fifty alternating 

 radii, with three or four fine filiform strise in each interspace ; 

 also crossed by fine irregular stride of growth. 



Two specimens measure, respectively, 19.5 ram. and 14 mm. 

 in length, and 23 ram. and 16 mm. in width, at the hinge. 



Incipient Shell (Plate XVII, figures 1, 1 a, 3). — The form 

 is nearly plano-convex. Dorsal valve convex on the umbo, 

 flat below. Veyitral valve moderately convex, with a promi- 

 nent pointed beak. Hinge narrow, with a small cylindrical 

 perforated pedicle-tube in the centre of the ventral area, and 

 a small callosity in the dorsal area. In the smallest speci- 

 men observed the surface is marked by eleven radii on the 

 ventral valve, but is otherwise apparently smooth. Length 

 2.25 mm.; width in the centre 3 rara. 



Developmental Changes. 



On account of the imperfection of tlie material, it is impos- 

 sible to trace any minor changes in the outline of the valves, 

 and the specimens indicate that no considerable transforma- 



