Speciale's "Historia Sicula" (in Muratori's Script.
They formed a separate system (the Rete Sicula) until in 1906, like the rest of the railways of Italy, they passed into the hands of the state, with the exception of the line round Mount Etna and the line from Palermo to Corleone.
A Monograptus makes its first appearance as a minute dagger-like body (the sicula), which represents the flattened covering of the primary or embryonic zooid of the colony.
This sicula, which had originally the shape of a hollow cone, is formed of two portions or regions - an upper and smaller (apical or embryonic) portion, marked by delicate longitudinal lines, and having a fine tabular thread (the nema) proceeding from its apex; and a lower (thecal or apertural) portion, marked by transverse lines of growth and widening in the direction of the mouth, the lip or apertural margin of which forms the broad end of the sicula.
A bud is given off from the sicula at a variable distance along its length.
This theca grows in the direction of the apex of the sicula, to which it adheres by its dorsal wall.
Thus while the mouth of the sicula is directed downwards, that of the first serial theca is pointed upwards, making a theoretical angle of about 180° with the direction of that of the sicula.
Each zooid buds from the one immediately preceding it in the series, and intercommunication is effected by all the budding orifices (including that in the wall of the sicula) remaining permanently open.
The sicula itself ceases to grow soon after the earliest theca have been.
In all these families the polypary originates as in Monograptus from a nema-bearing sicula, which invariably opens downwards and gives off only a single bud, such branching as may take place occurring at subsequent stages in the growth of the polypary.
2, Monograptus dubius, sicula and first serial theca (partly restored).
To four and these coalesce by their dorsal walls along the line of the nema, and the sicula becomes embedded in the base of the polypary.
The thecae in the earliest family - Dichograptidae - are so similar in form to the sicula itself that the polypary has been compared to a colony of siculae; there is the greatest variation in shape in those of the latest family - Monograptidae--in some species of which the terminal portion of each theca becomes isolated (Rastrites) and in some coiled into a rounded lobe.
It is the general practice of palaeontologists to regard each graptolite polypary (rhabdosome) developed from a single sicula as an individual of the highest order.
Certain American forms, however, which are preserved as stellate groups, have been interpreted as complex umbrella-shaped colonial stocks, individuals of a still higher order (synrhabdosomes), composed of a number of biserial polyparies (each having a sicula at its outer extremity) attached by their nemacauli to a common centre of origin, which is provided with two disks, a swimming bladder and a ring of capsules.
An exception, however,, is constituted by the comprehensive genus Dictyonema, which embraces species composed of a large number of divergent and sub-parallel branches, united by transverse dissepiments into a symmetrical cone-like or funnel-shaped polypary, and includes some forms (Dictyograptus) which originate from a nema-bearing sicula and have been claimed as belonging to the Graptoloidea.
The theory of the gonangial nature of the vesicular bodies in the Graptoloidea is, however, disputed by some authorities, and it has been suggested that the zooid of the sicula itself is not the 20, Climacograptus Scharenbergi.