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 A'^alve, Of these the plications near the 

 cardinal margin are low and incipient, but 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 tliicken- 

 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, ligures 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, 



