290 Raymond — Common Devonian Bracliiopods. 



Protegulum. — The shell at this stage is exceedingly small 

 and in none of the specimens of the present material are the 

 beaks well enough preserved to show the protegulum. From 

 Beecher's work it is known that in C. scitulus* the protegulum 

 is nearly circular in outline ('llY 111111 long and '111 111111 wide, accord- 

 ing to the figure), with a strongly arcuate hinge. 



Nepionic Stage. — The species agree in having the shell at 

 this stage convex in the ventral valve, convex at the umbo, and 

 concave or flat in front on the dorsal valve. This is an advance 

 on the condition in Stroj)heodonta, where the dorsal shell is 

 convex during the whole of the nepionic stage. 



The ventral valve always has a narrow sinus, and the dorsal 

 valve a corresponding median fold, with usually two less 

 definitely marked lateral folds. 



The outline is subcircular, though the width is frequently 

 a little greater than the length. The hinge is somewhat arcuate. 

 The length of the nepionic shell varies considerably in the 

 different species, but is always less than one millimeter. It is 

 least in C. coronatus and greatest in C. mucronatus. 



Figure 7. — Chonetes scitulus Hall; ventral and dorsal valves, showing 

 fold and sinus and plications of very young specimens, x 12. 



Pauciplicate Neanic Stage. — In this stage new striae are 

 introduced by implantation and not by bifurcation of the older 

 ones. The striae are strong, simple, and separated by spaces 

 as wide as the striae themselves. The number of plications is 

 not great. In C. coronatus and C. scitulus the largest number 

 is usually 13 on the ventral valve and 12 on the dorsal. In 

 the other species there are more, but usually less than 20. 



The shell is about one-fifth wider than long and gently con- 

 cavo-convex. The first spines, usually two or three pairs, 

 appear during this stage. The size of the shell at the end of 

 this stage, that is, when the striae begin to bifurcate, is least in 

 C. scituhis, when it is about 2 mm long, and greatest in C. robus- 

 tus, where a length of 6*5 mm is reached. (See specimens 

 Nos. 1-9, Plate XYI, Eow 3 ; Nos. 1-6, Plate XIV, Eow 1 ; 

 Nos. 1-7, Plate XV, Row 4; Nos. 4, 6, 7, Plate XVII, Row 1.) 



Later Neanic and JEphebic Stages. — It would be difficult to 

 make any sharp line which would mark the end of the adoles- 

 cent period and the assumption of all the adult characters. 

 The size, convexity of valves, and number of striae continue 



* This Journal, vol. xli, 1891, p. 357, pi. xvii, fig. 14. 



