18!K>.] NATURAL SCIENCES OF PHILADELPHIA. 393 



imeii which had been hatched a few days. This radula measured 

 0-72 nun. long, and was in the posterior part 0*28 mm. wide. It has 

 65 transverse rows of teeth numbering 7 - <r7 in front, IOScS^IO 

 in the posterior part ; so that the new longitudinal rows formed dur- 

 ing its growth are 11 on either side. The first transverse row of this 

 radula corresponds approximately to the twelfth in fig. I or VI, thus 

 the thirtieth to the forty-second, the forty-fifth to the fifty-seventh of 

 the whole series ; the teeth of about the 11 first formed transverse 

 rows were cast off. 



All teeth present in fig. II would become typical laterals in the 

 adult animal. They are primarily transverse bars ; at the inner end 

 of each bar the mesodont is formed, at first quite small, and then two 

 or threee ectodonts appear, which, as the mesodont grows, come to 

 stand on the latter ; (figs. II, 45: 13-11 ; III, 13) with the exception 

 of one which remains upon the common reflection, and is transformed 

 into the final ectodont. Of the endodont not a trace is present 

 originally ; and it forms only later when the tooth passes through 

 the intermediate or ' ' transition ' ' stage to become a true lateral. 

 The consequent stages may be seen in comparing the teeth with 

 corresponding numbers in figs. II-\ T ; thus tooth 15 is shown from 

 its beginning almost to full maturity ; and the figures give a better 

 idea of the changes than any description. A main factor in the 

 development of the tooth is the growth of the plate of attachment, 

 forward and backward from the "body" of the tooth. 



THE MARGINAL TEETH. 



The marginals, although much resembling the outer laterals in a 

 certain stage, are somewhat different from the first. Instead of being 

 a transverse bar, the base of attachment is longitudinally elongated, 

 as seen in figs. Ill, 2<S ; IV, 40, 41 ; and V 45, 46. The aculeate 

 mesodont is at first very small, but it grows rapidly, and is at once 

 directed obliquely inward. 



The marginals are not all alike. Those toward the inside are seen 

 quite early to bear ectodonts (fig. Ill, 20-27). The ectodont 

 becomes rather large, but subsequently disappears (fig. V, 25). 

 There is, however, a decided irregularity in this character, for in the 

 same longitudinal row there may be teeth with, and others without 

 an ectodont (fig. IV, 30). The distal marginals seem always to be 

 simple, without an ectodont (fig. IV and V). It may be noted for 

 26 



