I 1 6 NEW YORK STATE MUSEUM 



ways in becoming buried in the mud. Since this is the most perfect speci- 

 men of this species observed, it is quite possible that we have here before 

 us a part of the original antisicular end of the rhabdosome and that at that 

 end the branches were in contact. 



If, of the two genera Dicellograptus and Dicranograptus, we have good 

 reason to consider the former as the more primitive one and as having led 

 to the development of the latter, we are further induced to infer that this 

 development took place along various racial lines, this inference basing 

 itself on a number of similarities between species of the two genera which 

 find their expression partly in the like form of the thecae and partly in 

 the like habit of the rhabdosomes. Elles and Wood have divided both 

 genera in four subdivisions [here recognized] by the character of the 

 ventral walls of the apertures of the thecae. These groups run completely 

 parallel in both genera, i. e. we find in both all stages between forms whose 

 thecae have straight ventral walls and horizontal apertures and forms in 

 which tlie thecae have markedly curved ventral walls and strongly intro- 

 verted and introtorted apertural portions. The inference suggests itself 

 from these facts that a number of forms with primitive, little sigmoidally 

 curved thecae developed directly into Dicranograpti by the coalesence of 

 the sicular portion of the rhabdosome without further advancing the form 

 of the thecae. There exist likewise such similarities in the habits of the 

 forms of the corresponding divisions of the two genera, that their direct 

 phylogenetic connection can not be doubted. 



The genus Dicellograptus again is connected by many obvious bands 

 of relationship with Leptograptus, thereby indicating the derivation of the 

 Dicranograptidae from the Leptograptidae. This relationship shows itself 

 most distinctly in the thecae which in the development of the sigmoid 

 curvature and the depth of the excavation graduate l>v a continuous line' of 

 transitions from the Leptograpti with their shallow excavations and gently 

 curved thecae to the Dicellograpti of the first subdivision. In some forms 

 one is consequently in doubt as regards the character of the thecae, to which 

 of the two genera one is to refer it. Elles and Wood have also shown that 



