HISTORY OF RESEARCH. cxvii 



As regards the development of the proximal thecge, Wiman agrees in the main 

 with Tornquist's observations, but differs in the interpretation of the facts. 

 He regards the " connecting canal " as merely a bud which develops into the 

 proximal part of the first theca. In the special form of Diplograptus figured by 

 Wiman in illustration, the second theca develops directly from the first theca, and 

 each subsequent theca " develops from the next more proximally situated theca of 

 the opposite side." There is, therefore, no special structure such as a common 

 canal to give origin to the theca in this species, nor is there any double longi- 

 tudinal septum. 



This important paper was followed a few months later by 

 yyi ma/n another by the same author on the structure of Monograptus. 



" Ueber Monograptus, The structural details described in this paper are worked out 

 Geinitz," 'Bull, of the i n specimens of M. (iublus. 



Geol. Iustit, of Upsala, rpj ie s j cu ] a m ^j g Monograptus is shown to be essentially 



similar in structure to that in Dvplograptus; the mode of 

 development of the virgula is also similar. 



The growth lines at the point of origin of the first theca show a very marked 

 discontinuity; the passage between sicula and theca is wider than in Diplograptus, 

 and the first theca arises from the left side in the obverse aspect. This first theca 

 does not grow downward at first and then curve upward as in Diplograptus, but 

 grows upward at once. Each subsequent theca develops from the one immediately 

 below, and not from a common canal. Wiman observes the same zig-zag-like union 

 of the growth lines on the thecge as noted in the case of Diplograptus. 

 1893. 



Lake and Groom, 



" On the Llandovery In 1893 Lake and Groom noted the occurrence of the 



and Associated Bocks upper part of the Monog. gregarius zone (Lower Birkhill) in 

 of Corwen," ' Quart, t ] ie Corwen district of North Wales. 

 Journ. Geol. Soc,,' 

 vol. xlix. 



A short note on the structure of the skeleton of Mono- 



g ll as graptus priodon was published by Sollas in 1893. By means 



" On the Minute Struc- of transverse sections he was able to recognise in British 



ture of the Skeleton of specimens the three layers (comp. Richter, 1871) : " an outer 



Monog. pnodon, 'Geol. an( j mnei>j winch are very thin, separated by a space now 



filled with calcite, and a thicker middle layer." He notices 

 also and explains the thickening of the wall of the theca near its inner opening into 

 the common canal and also along the free edges of the theca, but points out that 

 this is caused " partly by an enlargement of the space between the layers, and 

 partly by a thickening of the middle layer." 



" The virgula," he holds, " would appear to possess no independent existence ; 

 it seems to be merely a thickening of the middle layer." 



