302 



THETIS SCIENTIFIC RESULTS. 



were fossil. Subsequent to his description of the species McCoj^ 

 recognised T. acuticostata as living in Bass Straits.* 



The series before me well illustrates the early stages of growth. 

 I had remarked on the descrepant sculpture these exhibit, when 

 my friend, Mr. T. S. Hall, informed me that he had simultaneously 

 discovered this interesting feature. He has since published! an 

 excellent account of this, showing how the ornament of Mesozoic 

 ancestors persists in the young of recent Triyonia. 



A young shell, 1-15 mm. long and 1-05 mm. high, is here shown 



(fig. 46). The prodissoconch is 

 smooth and inflated with a broad 

 up-turned rim. On the posterior 

 half of the valve are seven radiating 

 spinose ridges, alternately older and 

 younger, two spines corresponding 

 in age to one of anterior concentric 

 lamella, the interstices of the radii, 

 but not of the lamellae, are finely 

 granulate. At every second lamella 

 a fresh radius arises, and thus 

 retreating step by step the lamella; 

 shrink till at the tenth they disappear. I find that the brephic 

 stage here described of var. reticulata is precisely similar to that 

 of typical T. margaritacea and of its other varieties, viz., nobilis, 

 Ad., duhia, Sowb., lamarckii, Gray, and itniophora, Gray ; also to 

 that of T. strangei. All these forms also possess a small chondro- 

 phore immediately posterior to the umbo, of which I find no note 

 in literature. 



At its inception the hinge appears to consist of a cardinal and 

 a lateral in the right valve, each fitting into its respective sockets 



Trtgoxia margaritacea. 

 Fig. 47. 



Left Valve. Right Valve. 



BRKriiic stage ok hinge in Trigonia. 



Fig. 48. 



in the left. With growth (three stages shown in fig. 47) 



both cardinal and lateral add cog upon cog, advance and unite to 



form the adult complex. Corresponding modifications occur in 



* Crosse — Journ. de Conch., xxiv., 1876, p. 396. 



t Hall— Proc. Roy. Soc. Vic, xiv., 1901, pp. 17-'21, text Hg. 



