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NEW YORK STATE MUSEUM 



body which he termed the " initial point " or " radicle," and for ^v•hich 

 Lapworth later on introduced the terra " sicula." 



The details of the structure of the sicula have been elucidated by 

 Wiman, who has demonstrated that it consists of two very different parts 

 [see %.2, '2a, 2b]. The initial part is a smooth^, thin walled conical body that 

 does not show any transversal growth lines and tapers at one end into a 

 hollow tube, the neraa, while the other, wider end is directly continuous 

 with the apertural part of the sicula, which is the larger and thicker 



Fig.^Diplograptus sp. Sicula, 

 showing tlie characters of the apical 

 (embryonic) and apertural parts. 

 From the Baltic sea limestone of 

 Bornholm. x37 (Copy from Wiman) 



Fig. 2a 



Fig. 2b 



Fig. 2a, 2b. Obvei-se iiml i-c\ eree ^ lews iif the 

 voung rhabdosome of a dichograptid sliowing 

 the sicula, its two parts and the gemmation of 

 the first theca. From the Asaphus-llmestone 

 of Oeland. x27 (Copies from Wiman) 



walled portion ; is provided with growth lines and in form approaches a 

 theca. The sicula has for this reason been termed the first theca by Holm. 



The question appears pertinent, whether in the sicula itself (its apical 

 part) we have to see a product of sexual reproduction or of some form of 

 gemmation. This leads to the further inquiry after the organ of reproduction 

 of the graptolites. 



Hall believed he had found such organs in appendages of the 

 rhabdosomes of Diplograptus whitfieldi [1859, p.507], which 

 give the impression of macerated or ruptured vesicles. But no siculae were 

 found in connection or inclosed in them ; and a large series of specimens of 



'In Diplograptus provided with longitucjinal anastomosing thickenings. 



