116 



Guide to the Invertelrata. 



Sponges. 

 OALIEBT 



X. 

 West Side, 

 Wall -cases 

 7&8, 

 TaWe- 

 cases 

 11-16. 



simplest type the rays are equal in length and pointed, and the 

 spicule has the form of a caltrop (Fig. 1 69a, c). In other spicules one 

 ray is developed into an elongate shaft, from the summit of which 

 three short simple or furcate rays extend at varying angles, forming 

 a sort of trident. In other spicules the shaft-ray is reduced to 

 a small point, and the three other rays are extended and branched 

 horizontally (Fig. \md). Relatively large fusiform spicules are 

 associated with the four-rayed forms in these sponges. Some 

 sponges also possess a dermal layer or crust of minute kidney- 

 shaped, globate, and flattened, disc -like bodies. The spicules 



Fio. 169. — Various forms of siliceous spicules of fossil tetractinellid sponges, 

 (ff) Regular four-rayed or caltrop spicule of Pachastrella. (b) A micro- 

 spined spicule in which only three rays are developed. {c) Moniliform 

 four-rayed or caltrop, {d) Trifid spicule in which the shaft is rediiced, 

 and the head-rays are furcate and horizontally extended, [l) Caltrop spicule 

 of Pachastrella. {f-k) Trifid spicules showing variously modified head-rays, 

 a, X 26; i, X 40; c, X 54; <?, X 13; /, X 13; ^, X 26; A, X 13; 

 t, X 13; j, X 26; ^, x 100; I, x 13. 



in this group of sponges have generally a radial fascicular 

 arrangement, but they are not fixed together in any way, and 

 consequently they have for the most part been disintegrated in 

 the fossilization, though some entire forms, such as the Pacha- 

 strella convoluta, Hinde, from the Upper Chalk of Flamborough, 

 have been preserved. On the other hand, detached spicules of 

 these types are extremely abundant, forming the main constituents 

 of beds of sponge-rock in the Lower and Upper Greensand, and 

 also in the Upper Chalk. 



III. Lithistidae. — There is very great variability in the form 

 of the skeletal spicules in different sponges included in this 

 suborder, but they are alike in having the spicules intimately 

 connected' with one another by the intertwining and interlocking 



