FOSSIL INVERTEBRATES/Porifera 413 Figure Representative hexactinellid sponges (A) Twenhofelella, Silurian, Quebec, Canada (B) Hydnoceras, Devonian, New York, USA (C) Mattaspongia, Devonian, Alberta, Canada (D) Coeloptychium, Cretaceous, France (E) Diagoniella, Cambrian, Utah, USA All are natural size Adapted with permission from Rigby JK (1987) Phylum Porifera In: Boardman RS, Cheetham AH, and Rowell AJ (eds.) Fossil Invertebrates Palo Alto, Oxford, London: Blackwell Scientific Publications forms may or may not contain primary siliceous monaxon spicules, or pseudomorphs of such spicules, within their calcareous rigid elements Taxa within the polyphyletic group are differentiated by their shapes and chamber arrangements, their spicule content, as well as by the mineralogical compositions of their skeletons, internal segmentation, canal systems, and filling structures They range from simple beaded moniliform sponges to polyglomerate cylindrical forms, or to mound-like or stratiform multichambered sponges (Figure 7) Filling structures within chambers may include septae, vesiculae, spore-like elements, or fine reticular structures, amongst others, or the chambers may have been hollow and lacked such skeletal structures octactine spicules The small class has a geological record that extends from the Lower Cambrian to the Lower Permian, and is one of only two classes of sponges to have become extinct The Heteractinida includes such forms as the broad saucer-shaped octactinellid Astraeospongium (Figure 8), which is a common genus from the Silurian The class is particularly common in Silurian and Carboniferous rocks of Europe and North America Class Heteractinida de Laubenfels, 1955, Lower Cambrian–Lower Permian Order Octactinellida Hinde, 1887, Lower Cambrian–Lower Permian ?Order Hetairacyathida Bedford and Bedford, 1937, Lower Cambrian Heteractinida The Class Heteractinida, considered by some palaeontologists to be an order of the Calcarea, is characterized by skeletons made of large calcareous Sclerospongiae The Class Sclerospongiae was proposed to include a few living sponges, and is now thought by some