46 BRITISH FOSSIL SPONGES. 
Sponges are sub-spherical or pyriform, as species of Aulocopiwm, Melonella, and 
Siphonia ; club-shaped, as species of Phymatella and Rhopalospongia ; cylindrical or 
sub-cylindrical, as in species of Scytalia and Pachinion ; or dendritic, giving off 
branches from a main stem, as in Thamnospongia clavellata, Ben. ; or dichotomously 
branching with partial anastomosis of the branches, as in Doryderma dichotomum, 
Ben. Again, the branches may be compact, or merely traversed by longitudinal canals, 
as in the series just named, or they may be hollow tubes as in Sestrocladia furcatus, 
Hinde. Some Sponges are nearly spherical, as Astylospongia premorsa, Goldf., and 
Plinthosella squamosa, Zitt.; and others assume the form of mushrooms, as the 
lithistid Seliscothon planus, Phill., and species of the hexactinellid genus Celopty- 
chium. In other cases the growth of the Sponge is irregular, and the form in- 
definite, as in species of Plocoscyphia. 
In compound or colonial Sponges, the simple individuals forming the colony 
are more frequently cylindrical tubes, which spring from the same base, and form 
the mass either by subdivision or by budding. These tubes are either independent 
of each other, save where they start from the parent stock, or they partially 
coalesce together during their growth, and are only separate near their distal ends. 
It is difficult to determine in many cases whether the entire mass is to be considered 
as an individual Sponge, or as a group of individual Sponges growing together in a 
colony. ‘Thus, for example, in many species of Peronella the compound Sponge 
consists of cylindrical tubes growing parallel to, but separate from, each other, 
except at the point where their growth commences, and the Sponge is regarded as 
a colony of simple individuals. On the other hand, in Hlasmocelia faringdonensis, 
Mant., the cylindrical tubes are precisely similar in character to those of Peronella, 
and each probably has similar functions, but, instead of being separate, the walls 
are completely united together and enclosed in a common dermal membrane, and 
the entire mass is regarded as a simple Sponge, whereas strictly it is as much a 
colony of individuals as a compound example of Peronella. 
In many fossil Sponges the body of the Sponge is supported on a cylindrical 
stem or peduncle of varying length and thickness, and frequently having a minute 
structure differing considerably from that of the body itself. This feature is more 
particularly shown in lithistid Sponges, and good examples occur in the genera 
Chenendopora and Siphonia. It is not so prominent a feature in fossil hexactin- 
ellids, though well marked in Caloptychium. In general, fossil Sponges appear to 
have possessed flattened basal extensions, or elongated, branching, root-like 
appendages, for the purpose of anchoring themselves in the soft mud of the sea- 
bottom, or of attachment to other organisms or hard substances. These anchoring 
appendages take the form of horizontal or obliquely diverging extensions of the 
stem of the Sponge, or they may spring directly from the basal portion of the 
body of the Sponge in those cases in which no stem is developed. Such root-like 
