io6 



NEW YORK STATE MUSEUM 



that it points to an original dichotomous branching between each two suc- 

 cessive thecae, the production of the uniserial branch being only a secondary 

 acquisition, that insures greater strength to the rhabdosomes. 



In the Dicranograptidae the change in growth direction is still more- 

 complicated by one grade. Text figure 45 shows that the somatic cavity 

 changes its direction three times. At the budding point it diverges from 

 the mother theca but immediately reapproaches it, describing in its growth 

 a small semicircle ; then it adheres to the mother until close to the aperture 

 of the latter where it again diverges rather abruptly, but after again describ- 

 ing a part of a circle assumes a direction parallel to that of the free part of 

 the mother theca. The explanation of the first bending is, be ides the 

 above given reason, found in the desire of the young zooid to regain, after 

 having budded in a diverging direction, the mother theca as support in 

 further growth ; to the second bending it is clearly forced because it has to 

 give to the mother zooid free playroom for its apertural parts. This last 

 curvature causes the deep excavation in the theca. That this is formed to 

 get out of the way of the aperture of the mother, is clearly 

 seen in the fact that the first two thecae [see fig. 42] do not 

 possess it, because they, by bending away directly after bud- 

 ding from the sicula, are in their further growth not any 

 more encroaching upon the field of operation of other zooids. 

 Where the zooid becomes not only introverted, but also 

 introtorted with its distal part, the next theca is forced to 

 pass around in a correspondingly deeper curve and a deeper 

 excavation results. 



The dorsal margin of specimens retaining the periderm is 

 Fi K 6 Dicciio- nearly straight or shows but very slight undulations [see text 

 Lpworth. Fragment figr. 46 1 and therefore gives little indication of these rather 



preserved in relief. ° - 1 ° 



remarkable bends of the somatic cavities. The periderm has, 

 hence, been of considerable thickness on the dorsal side o\ the branch 

 and by this thickening there assumed the function of a supporting 

 axis. 



