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Thin sections of a compact, internal structure projecting from the hinge region of the bivalve Mya arenaria reveal the presence of tidal and seasonal patterns. This is the first demonstration that microgrowth increments are formed in a stucture not associated with the growing edge of the shell. The clarity and simple orientation of these extensive patterns, combined with the hinge region to disturbance and damage, suggests that detailed examinations of internal shell structures will prove valuable in ecological, archaeological, and paleoecological studies.

only qualitative data, but is easy to carry out and requires only a few common chemicals, whereas the procedure invented by Jeuniaux must be very laborious as it requires the isolation and purification of the enzymes chitinase and chitobiase. I believe the following results obtained by the chitosan method are valid qualitatively. Hydroids. It is generally accepted that the periderm of hydroids is chitinous. Tests made on the athecate hydroid Tubularia and the thecate hydroids Obelia and Aglaop/zenia are confirmatory ; their periderm gave a good positive test. Medusae. Hydrozoan medusa Polyorchis and scyphozoan medusa Linuche dissolved in the hot alkali and hence are devoid of chitin. Velelia. The sail of this well-known chondrophore is supported by an internal plate of cartilaginous consistency which gave a good color test. Nematocysts. Pieces of the tentacles of Physalia loaded with nematocysts dissolved completely in the hot alkali, an indication that the capsular wall of the nematocysts is not chitinous as sometimes thought. Entoprocts. Barentsia zorbunai and Barentsia disoreta dissolved completely in the hot alkali, hence are devoid of chitin, in sharp contrast with the ectoprocts which are mostly highly chitinous, except cyclostomes. Cyclostomatous ectoprocts. I previously reported a negative result with the one cyclostome available, Crisia eburnea. I have now made tests on the following species : Crisia eburnea, Crisia maxima, Crisina radians, Bicrisia edwardsiens, Crisulipora occiden talis, Filicrisia francanciscana, Diaperoecia californica, and Tubulipora tuba. All were negative. Jeuniaux reported a feeble content of chitin in Crisia denticulata and attributed my negative result to the less sensitive chitosan method. In any case the chitin content of cyclotomatous estoprocts must be very small in comparison with that of other ectoprocts. Brachiopods. Pieces of the shell of Lingula including the bristles gave a strong color test as previously known. An effort was made to recover the bristles, lost in the previous test ; they are strongly chitinous. In previous tests on Discinisca the bristles also were not recovered, hence a special effort was made this time to recover them; they also gave a strong color test.