Growth stages in modern Trigonias. 19 



The posterior slope has four or five radiating ridges, which at 

 first are smooth, but soon become nodulose, the most posterior 

 ones being the first to assume this character. As growth 

 proceeds, additional radiating ridges or costulae are intercalated 

 at varying distances from the prodissoconch. The concentric 

 ridges or costae on the anterior part of the shell eventually 

 become broken up into nodules, which later become radially 

 confluent and assume the character of the radiating nodulose 

 ridges of the adult, soon taking on their specific form. 



A usually strongly marked feature of the genus is the presence 

 of distinct ridge running from the umbo to the lower posterior 

 angle of the shell, and known as the marginal carina. In the 

 adults of the recent species this mai'ginal carina is comparatively 

 inconspicuous, but in the brephic stage of the present species, 

 T. margaritacea, its position is distinctly marked by a bold 

 radial ridge, which is more pronounced than those posterior to it. 



Just as the most dorsal of the posterior radiating ridges are 

 the first to assume the nodulose character of the adult, so the 

 concentric ridges give place to radial ones beginning with their 

 posterior ends, and the ancestral character of discrepant 

 ornament persists longest in the more anterior part of the shell. 



Carrying our observations a step further and passing by the 

 adult or ephebic stage — " the period of full development of the 

 individual, when all specific characters are clearly recognizable " 

 — we reach the gerontic stage of Messrs. Buckman and Bather, 

 or the senile of other authoi's, when " changes take place which 

 are due to a gradual failure of powers." In T. margaritacea the 

 nodules on the radiating ridges become more crowded and less 

 prominent, till finally they disappear among the increasingly 

 rugose lines of growth, while the radiating ridges themselves 

 also fade away and vanish. These signs of failing powers are 

 first shown in the siphonal area, and are here most strongly 

 marked ; while towards the anterior the nodules and ridges 

 persist much longer. 



In T. lamarckii the stages are similar, the brephic stage 

 showing discrepant ornament. 



In T. acuticostata the prodissoconch is like that of T. 

 margaritacea, but the concentric ridges of the brephic stage 

 persist till the shell has attained a breadth of 1-5 mm., and 



