1 64 



PORIFERA. 



by the possession of elongated spicules with bifid or trifid summits, 

 associated with a dermal layer of globate or reniform spicules. The 

 genus Pachastrella, again, comprises " massive, nodose, platter-shaped, 

 or irregularly expanded Sponges without a specialised dermal layer " 

 (Hinde). The skeleton is composed of four-rayed spicules, asso- 

 ciated with acerate spicules. The oldest species of Pachastrella are 

 found in the Carboniferous, but other forms of the genus have been 

 recognised in the Jurassic rocks, the Upper Chalk, and the Eocene 

 Tertiary. In the Upper Chalk occur the remains of various Tetrac- 

 tinellid Sponges, of which the genus Tethyopsis is perhaps the most 

 characteristic. In this genus (fig. 53), the skeleton is composed 



of radiately arranged trifid spicules, 

 mingled with acerates, while the 

 surface is covered with a layer of 

 minute trifid anchorate spicules 

 (Zittel). 



Order 5. Lithistid^e. — The 

 Sponges included in this order are 

 massive, stony, and thick - walled, 

 with a very variable external figure. 

 The skeleton is composed princi- 

 pally of four -rayed or irregular spi- 

 cules (fig. 54), which usually branch 

 at their extremities, their ends being 

 blunt, or being furnished with min- 

 ute articular surfaces. The skeletal 

 spicules are not fused together, but 

 are so interlocked " by the intertwin- 

 ing of their branches or by the close 

 apposition of their expanded extremities " (Hinde) as to give rise to 

 a practically continuous framework (fig. 54, b). There are generally 

 small monaxial " flesh-spicules," and also a dermal layer of trifid or 

 discoid spicules, in addition to the ordinary skeleton-spicules. There 

 may be a single terminal osculum only, or the Sponge may be 

 provided with numerous scattered oscula • while the canal-system 

 exhibits numerous modifications in different groups of the order, 

 and in some cases is not developed as a special structure. 



The order of the Lithistid Sponges has been divided by Zittel into the 

 four following sub-orders : — 



1. Rhizomorina. — In this group of Lithistids the skeleton-spicules are 

 elongated and irregularly branched (fig. 55, B), the branches terminating 

 in minute articular surfaces which unite adjoining spicules into an irreg- 

 ular and confused network. Typical genera are Cnemidiastrum, Selis- 

 cothon and Verruculina. 



2. Megamorina. — In this group the skeleton-spicules are large and 

 either simple or irregularly branched, the branches terminating in obtuse 



Fig' 53- — P art °f the skeleton of Te- 

 thyopsis Steinmanui, from the Upper 

 Chalk. Enlarged 14 times. (After 

 Zittel.) 



