118 



Guide to the Invertehrata. 



Sponges. The typical genus in this family is Doryderma, Zittel, which 

 OALLEEYis abundant in the Upper Greensand and Upper Chalk. 

 ''^. (<j) Ehizomorina. The skeletal spicule is relatively small, 

 «i „ usually elongate, curved and irregularly branching, with minute 



7 & 8, projecting spines. The branches terminate in minute lacets* 

 which are closely apposed to the axis and branches of adjoining 

 spicules so as to form an irregular mesh work or loosely- 

 arranged fibres (Fig. 170«, h). Typical genera are Verrucultna^ 

 Zitt., and Chenendopora, Lamx. 



Owing to the firm manner in which their skeletons are built 

 up, Lithistid sponges are abundant as fossils, and they are more 

 numerously represented in the Museum collection than the 

 Hexactinellids. They exhibit the greatest diversity in form and 



Table- 

 cases 

 11-15. 



Fig. 171. — Different forms of skeletal and dermal spicules of fossil lithistid 

 sponges. {a) Skeletal spicule of eutaxicladine lithistid, Mastosia. {b) 

 Skeletal spicule of anomocladine lithistid, Cylindrophyma. {c, d) Skeletal 

 spicules of tetracladine lithistid, Callopegma. {e) Skeletal spicule of tetra- 

 cladine lithistid Plinthosella. (/, g, h, i) Dermal spicules of lithistid 



o, X 40 ; *, X 40 ; c, X 40 ; <?, X 40 ; ^, X 26 ; /, X 40 ; ^, X 40 ; 

 A, X 40 ; i, X 40. 



size, from small rounded specimens not more than 6 mm. or \ inch 

 in diameter, to cylindrical or club-shaped examples 320 mm. or 

 13 inches in height and 105 mm. or 4 J inches in thickness. Cup-, 

 pear-, and platter- shaped forms are also very common. Numbers of 

 these sponges have, however, been broken up into their constituent 

 elements in fossilization, for their detached spicules are extremely 

 abundant in some of the beds of sponge-remains in the Upper 

 Greensand, and also in the interior of flints from the Upper Chalk. 



IV. Hexactinellidae. — In this suborder the characteristic spicule 

 has six rays, or arms, meeting in a common centre at right angles. 

 There is an axial canal in each ray, which connects at the centre 

 of the spicule with the canals from the other rays (Fig. 172a). 



