DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 329 



viclual. The first specimen in the ascending series which 

 shows the hinge distinctly has a length of 2.25 mm., and will 

 be described in the development of this part. 



The surface of the incipient shell is marked by seventeen 

 alternating, narrow, elevated radiating lines, with wider 

 interspaces, and also shows several lines of growth near the 

 margin. 



Developmental Variations. 



No marked changes occur in the general form of the shell 

 other than the gradual increase in the convexity of the dorsal 

 valve and in tlie extension of the cardinal angles. The 

 dorsal valve is usually quite fiat in specimens having a 

 length of 10 mm. or less. The radiating lines increase in 

 number by interstitial additions, from the youngest form to 

 maturity, and the fine concentric strice appear on all the 

 specimens, including the initial individual in the series, 

 where they are developed around the margins of the valves. 



The earliest phase of the hinge yet noticed is found in a 

 specimen having a length of 2.25 mm. The dorsal valve 

 shows a foramen in the cardinal area under the beak, mar- 

 gined by a slight thickening of the shell. The ventral valve 

 preserves a small perforate pedicle-tube at the apex, extend- 

 ing about two-thirds of the distance down to the hinge, below 

 which is a triangular deltidial opening of the same width as 

 the dorsal foramen. 



A specimen 4 mm. in length (Plate XVI, figure 19) shows 

 a more advanced development of the same parts. The dorsal 

 callosity has nearly filled the sinus under the beak and has 

 a narrow groove in the centre. The fissure of the ventral 

 valve has increased considerably in size and relative height, 

 showing narrow marginal plates or defined areas. The 

 pedicle-tube is still perforate, but has not increased in size 

 beyond the initial stage. 



From this point to maturity the hinge increases in width, 

 the dorsal callosity grows rapidly and nearly fills the fissure 

 of the opposite valve. The pedicle-tube is obscured, and the 



