402 NEW YORK STATE MUSEUM 



A difference of some diagnostic importance between the two genera may 

 rest in the presence or absence of a longitudinal septum. Hall, the author 

 of the genus Climacograptus, held that in this genus as in Diplograptus 

 the thecae were simple openings in the outer test of a single internal coeno- 

 sarcal canal. . Nicholson afterwards claimed that the rhabdosome of Climac- 

 ograptus consisted of two monoprionidian branches placed back to back, 

 and separated by a double median septum, consisting of the flattened dorsal 

 walls of the branches. Lapworth has later verified Nicholson's theory in 

 regard to C. scalar is and several of its varieties and C. wilsoni 

 and suggested that within the limits of Climacograptus, as at present con- 

 ceived, are included some forms in which the median septum is continuous 

 from side to side, and others in which the thecae of both series open into 

 one and the same central coenosarcal canal. Tornquist has shown in his 

 " Observations on the structure of some Diprionidae" [1893] by means of 

 thin sections that the species of Diplograptus investigated, had either no 

 median septum at all (D . b el lulus) or a very incomplete one (I), pal- 

 meus and C e p h al o g r a p t u s comet a where it is "reduced to a 

 narrow fold of the obverse periderm ") ; while in Climacograptus there are 

 first formed a series of alternating thecae from the common canal (Torn- 

 quist's " biserial chamber") as in Diplograptus, after which the biserial 

 chamber divides into two " uniserial chambers" which are separated by a 

 longitudinal septum. The number of pairs of alternating thecae is different 

 in different species ; C . s c a 1 a r i s was thus found to have two of them, 

 while in C. rectangularis the septal line in the reverse aspect does 

 not begin until beyond the eighth pair of thecae ; and in C . undulatus 

 the zigzagged septal groove begins at the sicular end. 



Almost simultaneously with Tornquist Wiman made his investigations 

 on etched material of Diplograptidae [1893 and 1895]. He showed the 

 origin of such a longitudinal septum in Climacograptus [see text fig. 353] 

 by means of a monopodial branching of a theca, the third theca (in C. 

 kuckersiana) sending out two thecae, first one (t_|, d in figure) toward 

 the side of the second theca, and then one (ts) towards the side of the first 



