Among the recent genera Penicillus and Codium may be chosen as important types from the point of view of fossil representatives.
Codium.
The thallus of Codium consists of a spongy mass of tubular cell-branches which are differentiated into two fairly distinct regions, an outer peripheral layer in which the branches have long club-shaped terminations, and an inner region consisting of loosely interwoven filaments.
Codium Bursa L. and C. tomentosum Huds. are two well-known British species, the former presents the appearance of a spongy ball of cells, and in the latter the thallus is divided up into dichotomously forked branches[280]. In this genus the thallus is not encrusted with carbonate of lime, at least in recent species.
Sphaerocodium. Fig. 37, D.
Rothpletz[281] instituted this genus for certain small spherical or oval bodies varying from 1 mm. to 2 cm. in diameter, which have been found on crinoid stems or shell fragments of Triassic age. Each spherical body consists of dichotomously branched single-celled filaments, between 50 and 100µ in breadth, and from 300–500µ in height. The tubular cavities occasionally swell out into spherical spaces which are regarded by Rothpletz as sporangia.
There is not sufficient evidence that Sphaerocodium Bornemanni Roth. has been correctly referred to the Codiaceae. The sporangia-like swellings described by the author of the species are not by any means conclusive as characters of important taxonomic value. Figure 37, D, illustrates the general structure of the fossil as seen in a transverse section of one of the calcareous grains.
Like Girvanella, which has been referred by some writers to the Siphoneae, Sphaerocodium occurs in the form of oolitic grains. In the Triassic Raibler and St Cassian beds of the Tyrol, as well as in rocks of Rhaetic age in the Eastern Alps, it makes up large masses of limestone. Rothpletz compares the structure of this genus with that of the recent alga Codium adhaerens Ag., but it is wiser to regard such tubular structures as Girvanella, Siphonema[282] and Sphaerocodium as closely allied organisms, which are probably algae, but too imperfectly known to be referred to any particular family.
Penicillus.
The recent genus Penicillus is one of those algae formerly included among animals. Fig. 33, O, has been copied from a drawing of a species of Penicillus given by Lamouroux[283] under the generic name of Nesea in his treatise on the genera of Polyps published in 1821. He describes the genus as a brush-like Polyp with a simple stem.