The hypothecium is the basal part of the apothecium on which the hymenium is borne; the latter consists of asci (thecae) with ascospores, and paraphyses.
B, Asci (thecae) with bilocular spores.
There is a tendency for the branches to become reduced in number, and for the serial thecae to become directed more and more upwards towards the line of the nema.
The thecae uniserial (monoprionidian) - t here is a gradation from earlier groups with many branches to later groups with only two; and from species in which all the branches and' their thecae are directed downwards, through species in which the branches become bent back more and more outwards and upwards, until in some the terminal thecae open almost vertically.
The Monograptidae, the branches are theoretically reduced to one, the polypary is uniserial throughout, and all the thecae are directed outwards and upwards.
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.
The thecae in several of the families are occasionally provided with spines or lateral processes: the spines are especially conspicuous at the base in some biserial forms: in the Lasiograptidae the lateral processes originate a marginal meshwork surrounding the polypary.
In Dictyonema the branches show thecae of two kinds: (I) the ordinary tubular thecae answering to those of the Graptoloidea and occupied by the nourishing zooids; and (2) the so-called bithecae, birdnest-like cups (regarded by their discoverers as gonothecae) opening alternately right and left of the ordinary thecae.
Internally, there existed a third set of thecae, held to have been inhabited by the budding individuals.
In the genus Dendrograptus the gonothecae open within the walls of the ordinary thecae, and the branches present an outward resemblance to those of the uniserial Graptoloidea.