66o F. C. GREENE 



STAGES IN DEVELOPMENT 



As in any series, the division into stages is more or less arbitrary, 

 but there are several factors which determine approximately the 

 different stages. In the series of ventral valves (Plate I, rows i and 

 2) it will be seen that all stages prior to specimen 14 (point A in the 

 plate) are not marked with striae and have only the one spine (Plate 

 IV, 1-3, 6-8) while beyond this point the striae appear and increase 

 in number regularly (Plate IV, fig. 5) . This point is taken to mark 

 the end of the nepionic and the beginning of the neanic stage. The 

 latter stage continues until the shell takes on its adult characters, 

 such as the number and strength of the striae, the shape of the cardinal 

 angles, a widening of the hinge area causing the beak to project and 

 the hinge to appear bent. The point B about marks this division 

 although it cannot be drawn with the same precision as the division 

 between the first two stages. 



The last two (and largest) specimens in the series represent the 

 old age or gerontic stage. The intervening specimens represent the 

 adult or ephibic stage. 



Another series is made to show further the last two stages. This 

 is the series of specimens of the interior of the dorsal valve (Plate II, 

 rows 3 and 4) . Here the last specimen represents the gerontic stage. 

 This shows (i) a great thickening of the shell; (2) prominence of the 

 muscular and brachial markings; (3) greater length of hinge-line; 

 (4) a lesser number of granules within the visceral cavity. These 

 characters in this brachiopod are indications of old age. 



COMPARISON OF THIS SERIES WITH SPECIMENS OF THE GENUS 

 CHONETES, OLDER AND YOUNGER GEOLOGICALLY 



It will be noted from the measurements discussed under "Devel- 

 opmental Changes," that the length and breadth of the shell of 

 the nepionic stage are about equal. This is true of practically all 

 the Silurian and early Devonian species of Chonetes in the adult stages. 



The acceleration in the development of this species is illustrated 

 by the appearance of a pair of spines in the nepionic stage in specimens 

 less than i mm in diameter instead of in the neanic stage as in 

 C. scitulus, where they first appear on specimens about 2 mm in 



