20 



MEMOIRS OF THE STATE MUSEUM 



from the pedicle-tube to the extremities. Surface smooth, except along 

 a narrow zone around the margin, which shows incipient radiating striae. 



DEVELOPMENTAL CHANGES 



The form of this species being somewhat complex, we may make a 

 convenient subdivision of the development of the shell into four stages, 

 which may be briefly characterized as follows : 



isi Stage. Length of shells .4-1 mm.; surface smooth, 



2d.Stage. Length 1-2 mm.; shell radiatingly striated, without undulations. 



jd Stage. Length 2-20 mm.; shell radiatingly striated, and concentrically 

 undulated. 



^th Stage. Length 20-30 mm., entire shell radiatingly striated, concen- 

 trically undulated in the upper part, abruptly produced 

 and geniculated in front. 



The changes taking place in the form and character of the shell 

 from one stage to another can best be shown and used for comparison 

 in the following tabulation, where the conditions incident to each stage 

 of growth in the various parts of the shell are briefly described. 



Development of St^'ophomena rhoniboidalis 



ist stage. 

 Initial. 



2d Stage. 

 Infantile. 



3d Stage. 

 Adolescent. 



4th Stage. 

 Mature. 



Size. 



.4 mm. — I mm. in i mm. — 2 mm. in 2 mm. — 20 mm. in 20 mm. — 30 mm. in 

 length, .4 mm. — length, 1.5 mm. — length, 2.5 mm. — length, 40 mm. — 

 1.5 mm. in width.' 2.5 mm. in width. 40 mm. in width. 50 mm. in width. 



Form. 



Transversely semi- Transversely semi- Longitudinally. Longitudinally 

 oval; cardinal ex- elliptical; cardi- semi-elliptical; semi-elliptical; 



tremitiesobtusely 

 angular. 



nal extremities 

 angular. I 



cardinal extrenii- cardinal extremi- 

 ties angular, be- ties acutely angu- 

 coming produced, lar, extended and 

 not twisted. \ twisted. 



