42 ART. 4. N. YATSU : 



Fig. 85.), 8 p. c. (PI. VI., 84.), 12 p. c. (PI. VI., Fig. 87.) and 

 15 p. c. (PI. VI., Fig. 8G.) will afford a general idea of develop- 

 mental changes, and show how our Lingula-l&TVBô differ from 

 those figured by Brooks in having the narrower arm-apparatus, 

 a broader body proper and a shorter peduncle. 



b. Shell. 



During the 6-7 p. c. stage the shell is almost circular. It, 

 however, soon, increases more rapidly in length than in breadth, 

 and at the 8-9 p. c. stage it has become oblong : 



Length. Breadth. 



5 p.c. stage 313/;. 386//. 



6 „ „ 411/;. 444 ;x. 

 7-8 „ „ 663 li. 615 /;. 



The ratio of the hinge line to the length of the shell at 

 different stages is as follows : 



Here the hinge line is taken as 1, its actual length being 

 280 ft on an average. It should here be noted that in the young 

 Lingula of 5 mm. (in shell length) the ratio is 1 : 18.51. 



While at the 5-7 p. c. stage the shell is transparent and 

 colorless (PI. V., Figs. 77, 78, 79., and PL VI., Fig. 83.), at 

 the 8-9 p. c. stage the posterior and lateral margins of the shell 

 acquire a light brownish tint, and near the hinge line the 



