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 time to strengthen it and provide air-chambers to Iduov up the shells. 



A similar periodicity in the growth of the sliell is shown in the succes- 

 sion of flaring apertures of the Devonian Rli ifliceras cijclops (Hall) and 



Figure 7. — GcisouM eras 

 cf. transversum (Miller) 



View of portion of com- 

 pressed conch, showing 

 growth - lines and sutures 

 (dotted). X U. 



