io CALCAREOUS ALGAE OF THE MIDDLE EAST 



pores the lateral branches and the cavities the sporangia. There may be a terminal 

 " pepper pot " structure at the apex. Determination of the fossil is made on the 

 detail, number, arrangement and size of these structures. The essential prepara- 

 tions are a longitudinal cut or thin-section through the long central cylinder-axis, 

 and a transverse section at right angles to this. More than one of the latter may be 

 required when the lateral branches are strongly inclined relative to the horizontal 

 plane. A whole, three-dimensional specimen, or at least a weathered surface to show 

 surface-detail, is desirable for describing new material. A selection of such loose 

 specimens for the preparation of orientated sections is ideal, but not always available. 

 Once the essential structure is understood, the species may be recognized with 

 practice in all manner of random oblique or tangential cuts. It should be noted that 

 in exceptionally large dasyclads individual longitudinal and transverse sections, even 

 if correctly orientated, may pass between verticil-detail and so not be diagnostic. 



Exact measurements of the dimensions of thallus, stem-cell, branch- and 

 sporangial-detail are desirable in describing dasyclad algae. It should, however, be 

 remembered that whilst species do show average size and proportions of both thallus 

 and component structures, they are plants and, like all plant life, variable. Dimen- 

 sions are therefore a guide and not an exact character in dasyclad taxonomy. 



A considerably greater variety of form has existed in dasyclads through geological 

 time than survives at the present day. The notes which follow deal with the 

 principal types which occur. 



The stem-cell may be thin- or thick-cylindrical, club-shaped with either gradual 

 increase of diameter or bulbous termination or greatly swollen, even to a near- 

 spherical shape. Normally it is represented in fossils by a simple central mould, but 

 certain cylinders whose walls are formed by hollow spherical calcareous bodies, 

 adjacent, touching or fused, e.g. Atradyliopsis, are interpreted as remains of dasy- 

 clads in which the sporangia were within the stem-cell itself and calcified as a sub- 

 dermal stem-cell peripheral zone to give the fossil seen. Occasionally there is some 

 doubt as to whether a calcareous filling between these bodies occurred during the 

 life-time of the plant, in the lower, older part, or is a post-mortem mineralisation 

 feature (e.g. Aciculella) . 



Exceptional forms in which the stem-cell is creeping in habit, or modified into a 

 thin support for a large terminal disc or discs, are dealt with separately below. 



The lateral branches have been described above as occurring at successive hori- 

 zontal levels in whorls or verticils. Pia (1920) recognized a tripartite classification 

 of their relationship to the stem-cell. In the more primitive aspondyl type the pores 

 marking the origin of branches occur irregularly, more or less over the whole stem- 

 cell surface. In the euspondyl type single branches are set in approximately 

 horizontal whorls, whilst in the metaspondyl arrangement tufts of branches originate 

 from the pores of the verticils. 



The lateral branches themselves may be single structures, or more commonly 

 branched, so giving primaries, secondaries, tertiaries etc. The number of sub- 

 branches at each point of branching or division is approximately constant within the 

 species. These points of branching are constricted and there also occur genera in 



