A CARBONIFEROUS BRACHIOPOD 659 



number of spines increases about every eighth or ninth specimen, 

 until it reaches the maximum of eleven on one side of the beak, or 

 twenty-two on the entire hinge-line, which was the largest number 

 noticed. 



The shape of the youngest specimen is nearly square, while in the 

 oldest the breadth is nearly twice the length. As will be seen by the 

 above table of measurements, this change from one to the other is 

 somewhat irregular. 



The angle formed by the hinge-line and the sides of the shell 

 gradually changes from an angle of 90° to a more and more acute 

 angle, although, as has been pointed out before, the angle varies 

 greatly in the adult (Plate I). 



Dorsal valve. — The striae do not appear distinctly on this valve 

 until the twenty-fifth specimen, (4.3 mmX2.5 mm) but this is 

 probably due to the fact that some of the earher stages have been 

 omitted from this series, and that some are incrusted in the concavity 

 or water-worn, which renders the striae invisible in them. 



The most notable thing about the developmental changes in this 

 valve concerns the small convexity mentioned in the description of 

 the incipient form. In the smallest specimen this occupies one-half 

 the space of this valve and extends from the hinge-hne forward 

 three-quarters of the length of the shell. In the twenty-eighth 

 specimen it is crowded well up under the hinge-line and is so small in 

 comparison to the size of the shell, that it is hardly distinguishable. 

 In the oldest forms, it is so close up under the hinge-Hne as to be 

 entirely obscured. 



Interior of dorsal valve. — In the interior of this valve, the granules 

 in the visceral cavity continually become fewer as the specimens 

 become larger after the beginning of the ephibic stage. The muscu- 

 lar and brachial markings also become stronger as the size increases. 



DEVELOPMENT 



As a complete series was obtained from the Florena shales at 

 Florena and Grand Summit, this is used as a basis of comparison. 

 A complete series might have been obtained from Florena alone, but 

 as the adults were compressed. Grand Summit specimens were 

 substituted for them. 



