DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 345 



and incurved beneath the ventral foramen. A strong median 

 fold corresponds in development with the median sinus of the 

 opposite valve. 



Surface marked by strong, simple, sub-angular plications, 

 invariably two upon the fold and one in the sinus, with six 

 on each of the latera, making thirteen on the ventral and 

 fourteen on the dorsal valve. Of these the plications near the 

 cardinal margin are low and incipient, Ijut the full number 

 becomes permanent early in the history of the individual. 

 Faint concentric growth-lines are sometimes visible. Dimen- 

 sions of average adult 11 X 11 mm. 



Abnormalities at Maturity. — The variations from the nor- 

 mal mature form are, as far as observed, wholly due to con- 

 tinued internal growth after individual maturity has been 

 attained, and this is to be regarded as the concomitant evi- 

 dence of senescence. There may be either a marginal thicken- 

 ing, which gives the shell a truncate appearance, or a general 

 internal thickening, making the shell unusually gibbous, and 

 forcing the ventral beak over upon the dorsal umbo. 



Incipient Form (Plate XVIII, figures 1, la). — The young- 

 est individual observed measures 2.75 X 2 mm. ; outline sub- 

 ovate, valves regularly rounded, the ventral being the more 

 convex. Ventral valve with an erect, straight beak; apex 

 acute, cardinal margins sloping rapidly forward, and slightly 

 excavate. Foramen simple, triangular, free from deltidial 

 plates, encroaching at its apex slightly upon the umbo; for- 

 aminal margins somewhat thickened. Dorsal beak erect but 

 inconspicuous, full and rounded. Dorsal valve depressed 

 anteriorly along the median line, this depression correspond- 

 ing with the broad and low dorsum of the opposite valve. 

 Surface of each valve marked by eight single, rounded plica- 

 tions, which extend two-thirds the distance from the anterior 

 margin of the beak, leaving the circumbonal area smooth. 



Developmental Variations. 



General Form and Outline. — As growth advances, the de- 

 velopment is more rapid transversely than longitudinally, 



