DEVELOPMEXT OF SUME SILURIAN BUACIIIOPODA 307 



represents all developmental stages between the limits of 

 these dimensions: 2 x 1.6 mm. (incipient shell) and 7 X lj.5 

 mm. (maturity). In its youngest stages it shows a certain 

 degree of similarity with Homoeospira evax, especially in the 

 sinus on both valves and in the sinal plications. The 

 greater number of the latter in H. evax, as well as the more 

 numerous lateral plications, will serve to obviate confusion 

 here. 



Specific Characters. 



Mature Form (Plate XIX, figures 11, 11 a, 11 5). — Shell 

 small, rotund, in outline broadly ovate to sub-pentagonal. 

 Valves of equal convexity. 



Ventral valve with umbo prominent, attenuate, erect, and 

 slightly incurved at the apex ; cardinal margins not excavate, 

 sloping with a faint curve to the sides, whence they round 

 to the anterior edge, which, in the sinal region, is nearly 

 straight; cardinal area distinct; foramen circular; deltidial 

 plates prominent. 



Dorsal valve sub-circular in outline, arched in the urabonal 

 region; beak well-defined, apex concealed. In the umbonal 

 region the median portions of both valves are slightly more 

 convex than elsewhere, but this prominence disappears toward 

 the margins, the valves becoming slightly flattened and de- 

 pressed on the median region near the anterior margin, mak- 

 ing a low sinus on the ventral, and a low, depressed fold on 

 the dorsal valve. Both fold and sinus may bear one, two, 

 or three small, often faint and unsymmetrically developed 

 plications, the strongest of which may have its origin in the 

 umVjonal region, while the others rarely extend more than 

 half-way across the shell. On each of the latera are four 

 or five strong, angular, simple plications, making thus from 

 nine to thirteen plications on each valve. The increase in 

 these takes place altogether on the fold and sinus, the full 

 quota of lateral plications appearing early in the history of 

 the individual. The plications are covered by numerous 

 fine, concentric growth-lines more noticeably developed near 

 the margin, and at intervals becoming varicose. 



