Journal of conchology . HYALINIA. 23S difficulties of accurate measurement, which have already been noticed,render this conclusion uncertain, though it hardly seems likely thaterrors of observation can have led to such a large difference. Withregard to all the figures it must also be remembered that the shellsfrom which the radul^e were derived were not all the same size butranged either way from their mean by 6%, and that every item varieswith the size of the shell. ^ The two teeth which obviously vary in their configuration are thethird lateral and first marginal. With regard to the third la
Journal of conchology . HYALINIA. 23S difficulties of accurate measurement, which have already been noticed,render this conclusion uncertain, though it hardly seems likely thaterrors of observation can have led to such a large difference. Withregard to all the figures it must also be remembered that the shellsfrom which the radul^e were derived were not all the same size butranged either way from their mean by 6%, and that every item varieswith the size of the shell. ^ The two teeth which obviously vary in their configuration are thethird lateral and first marginal. With regard to the third lateral wemay distinguish five types, shewn in the diagrammatic sketch, whichoccur with the following frequency :— Type I. Endocone absent, ectocone absent Type II. Endocone small, ectocone absent Type III. Endocone small, ectocone a shoulder ... Type IV. Endocone present, ectocone absent Type V. Endocone present, ectocone a shoulder Type VI. Endocone present, ectocone present ... 0=0% 10=15% 6= 9% 10=15% 34 = 51% 6= 9%. SO ^ Type I. did not occur in the series. In the whole series the endo-cone was small in 24% of the specimens, the ectocone absent in30%, represented by a shoulder in 60% and well developed in 9%.With regard to the first marginal, the endocone was present in sevencases, though always small or rudimentary, and in two on only one-half of the radula. It should also be noted that in six of thesespecimens, the third lateral had a definite ectocone; in the other theectocone was represented by a shoulder only. This simultaneousprogress in differentiation is a regular feature of the Hyalinia an ectocone on the third lateral is commonly associated with awell-developed endocone, and if the latter is very small the ectoconeis often completely absent. The difference may sometimes be seen I The sort of difference which is introduced by this factor may be judged o: by the a\figures of half millimetre groups :— Group Mean diameter Rows Marginals Long Wide 7o—7
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Keywords: ., bookauthorconcholo, bookcentury1800, bookdecade1870, bookyear1879
