624 N'EW YORK STATE MUSEUM 



and that also the stem internodes between two bifurcations consist of but 

 one theca each; that hence the entire rhabdosome from the sicula on is 

 built up of thecae ; and that there are no indenticulate branches. There 

 exist however morphologic and functional differences between the earliest 

 thecae, which are constituent parts of the stems and the later thecae of the 

 branches, differences which also appear within the denticulate branches 

 themselves. These differences were held to indicate ontogenetic stages of 

 the nature of those which have been termed " localized ontogenetic growth 

 stages " by Jackson. Here the localization appears in so far as each branch 

 passes in the shape and arrangement of its thecae, like an organ of the 

 whole rhabdosome, through ontogenetic stages, indicative of phylogenetic 

 or evolutionary stages passed by the species. 



A single specimen [pi. 6, fig.l4] was obtained which had not been 

 spread out like the others on the bottom of the sea, but became compressed 

 laterally. This shows that all branches were held in a nearly horizontal 

 position, curving slightly upward in the distal parts^- and that the thecae 

 pointed all in one direction, viz downward, assuming the siispended 

 position of the rhabdosome. Another specimen has been figured here 

 because it exhibits an abnormal irregular branching on one (left) side 

 [pl.6, fig.l5]. 



Fragments of rhabdosomes Avhich have been stripped of a part of 

 their branches, are liable to assume very misleading aspects. Specially 

 numerous were specimens retaining only four branches, in such a manner as 

 to suggest a Tetragraptus [pl.6, fig.l3]. These, then, in the character of their 

 branches and thecae, are somewhat similar to Tetragraptus hicksii 

 Hopk. & Lap. [1875, p.651], a species which has lately been recognized to be 

 based on a bundle of rhabdosomes of an Azygograptus. 



^ The drawing has been inverted by mistake. 



