500 



MISS JOAN B. PROCTER ON THE 



very early stage, only the merest traces of bone-deposit are 

 present in place of the rib-like arms of these plates [vide text- 

 fig. 10). 



Fourth costal plates [cp. 4) in form precisely like second, twice or 

 more than twice width of third, sometimes with veiy ragged edges ; 

 widely separated from neural by third median fenestrse, forming 

 a suture with sixth marginal plates, sometimes with corner of 

 seventh also. Fenestra between third and fourth similar in size 

 and shape to that between first and second, about twice width of 

 that between second and third. 



Fifth costal plates {cp. 5) precisely like third in every way, 

 connecting by suture with seventh marginal plates. Fenestra 



Text-fiaure 10. 



Carapace of adult arrested at an earlier stage in development. 

 (Left side, from within ; J nat. size.) 



between fourth and fifth similar to that between second and 

 third, about half width of that between third and fourth. In 

 the poorly developed specimen already referred to (text- fig. 10) the 

 rib-like limb is represented by a few bone-granules in isolated 

 patches. 



Sixth costal plates {cp. 6) similar to second and fourth, but 

 rather more developed. Anteriorly narrowly in contact with fifth 

 beneath the vertebro-costnl shield-sutures, otherwise separated 

 by fenestra; slightly narrower than any of the preceding. Central 

 head in closer proximity to the neurals than those of second 

 and fourth; the dividing fenestra about half as large as in 

 the former instances. Posterior edge quite straight, forming 



