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NEW YORK STATE MUSEUM 



body wliicli lie termed tlie " initial point " or " radicle," and for which 

 Lapworth later on introduced the term " sicula." 



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

 Wimau, who has demonstrated that it consists of two veiy different parts 

 [see fig.2, 2a, 2b]. The initial part is a smooth^ thin Availed conical body that 

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

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

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



Fig. 2 D i p 1 o g r a p t u s sp. Sicula, 

 showing the characters of the apical 

 (embryonic) and apertural parts. 

 From the Baltic sea limestone of 

 Bornholm. x37 (Copy from Wiman) 



Fig. 2a, 2b. Obverse and reverse ^•iews of the 

 young rhabdosome of a dlchofiraptid showing- 

 the sicula. Its two parts and the gemmation of 

 the first theca. From the Asaphus-limestone 

 of Oeland. x2T (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, v^'hether in the sicula itself (its apical 

 part) we have to see a pi'oduct of sexual reproduction or of some form of 

 gemmation. This leads to the further inquiry after the organ of re2')roduction 

 of the graptolites. 



Hall believed he had found such organs in appendages of the 

 rhabdosomes of Diplograptus whitfield'i [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 longitudinal anastomosing thickenings. 



