. Bulletin of the Geological Society of America. Geology. DIFFERENTIATION IN SIZE OF SEXES 319 While there already exist observations made by European scholars on sexual dimorphism among the Mesozoic ammonites, we are not aware of such regarding the Paleozoic nautiloids. Barrande noted the wide differ- ences in the relative lengths of the living chamber in the same species of OrthoceraSj but saw in them but individual variations in size. We con- sider it very probable that they also denote a difference in the size of the sexes. Synchronic relation of Septa to Growth-lines A further observation
. Bulletin of the Geological Society of America. Geology. DIFFERENTIATION IN SIZE OF SEXES 319 While there already exist observations made by European scholars on sexual dimorphism among the Mesozoic ammonites, we are not aware of such regarding the Paleozoic nautiloids. Barrande noted the wide differ- ences in the relative lengths of the living chamber in the same species of OrthoceraSj but saw in them but individual variations in size. We con- sider it very probable that they also denote a difference in the size of the sexes. Synchronic relation of Septa to Growth-lines A further observation on an Ordovician species of cephalopods relates to the origin of the septa. A species of Orthoceras, namely, 0. (Geisonoceras) cf. transversum Miller, has been found in the lower Lorraine shale preserving (on speci- mens that are compressed into a single plane and whose substance is totally dissolved) in the same plane both the septal sutures and the sur- face sculpture. It is seen in these specimens that the transverse growth-lines of the sculpture are crowded at regular intervals, and that these intervals exactly correspond to the depth of the camerse or to the distances between the septa as indicated by the sutures. Assuming that the crowded growth-lines denote periods of rest in the growth of the animal, it follows that the septa likewise indicate regularly returning periods of rest folloAving times of rapid growth. These periods of rest are believed to correspond primarily to the periods of reproduction in the mature specimens. The i)eriods of rest and sexual production, in which the septum was formed, alternate regularly with periods of rapid growth and lengthening of the shell. The formation of the septa, which originated with the periodic interruptions in or a slackening of the rate of growth in the mature individuals, has l)y tach3^genesis been carried back into the earlier growth-stages for the purpose of shutting off the unused portions of the conch and at the same
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