46 BRITISH FOSSIL SPONGES. 



Sponges are sub-spherical or pyriform, as species of Aulocopium, Melonella, and 

 Siplwnia; club-shaped, as species of Phymatella and Bhopalospongia ; 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 Dory derma dichotomum, 

 Ben. Again, the branches maybe compact, or merely traversed by longitudinal canals, 

 as in the series just named, or they may be hollow tubes as in Sestrocladiafurcatus, 

 Hinde. Some Sponges are nearly spherical, as Astylospongia prcemorsa, Goldf ., and 

 Plinthosella squamosa, Zitt. ; and others assume the form of mushrooms, as the 

 lithistid Seliscothon planus, Phill., and species of the hexactinellid genus Ccelopty- 

 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 Dlasmocoelia faring donensis, 

 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 Goelopty chium. 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 



