12S M. K. A. Zittel on Fossil Lithisttdce. 



essentially skeleton-forming elements of the Lithistidae, which 

 constitute the principal mass of the sponge-body. These cor- 

 puscles are very seldom truly spicular in their form ; they are 

 never simple, straight, and pointed at one or both ends ; but 

 they are always composite, more or less branched corpuscles, 

 and usually furnished with root-like appendages, which pre- 

 sent but little resemblance to the siliceous skeletal elements of 

 other sponges. I therefore regard it as inadmissible to speak 

 of " skeleton-spicules " in the Lithistidee, and will adopt the 

 name of "skeletal elements" or "skeletal corpuscles" for 

 them. 



On the whole, a great uniformity prevails among the Lithis- 

 tidae with regard to the form of their skeletal corpuscles ; so 

 that it is only exceptionally that they suffice by themselves 

 for the characterization of a genus. 



In the most strongly marked and probably also highest 

 Lithistidae, almost all the constituents of the skeleton, both 

 the true skeletal corpuscles and the surface-spicules, are quadri- 

 radiate, which, however, does not prevent one of the four rays 

 being developed differently from the other three. I denomi- 

 nate this group Tetracladina. If we place any fragment 

 of the skeleton of a Phymatella^ Siphonia, Cailopegma, Aulaxi- 

 nia, Turonia, Jerea, &c. under the microscope, it is seen to be 

 composed entirely of quadriradiate corpuscles similar in form 

 and also agreeing pretty closely in size (PI. VIII. fig. 1). All 

 the four arms, of equal length, meet in the centre at angles of 

 120°; they are usually smooth, rarely beset with verruciform 

 excrescences, and divided at the ends turned away from the 

 centre into a few short branches, which again may be in turn 

 beset with root-like processes. According as the four arms 

 divide first of all into two or more thick branches, and these 

 again into finer ramifications, or even into short fibres, there are 

 produced at the ends pad-like dilatations composed of small 

 root-like fibres. When well preserved, we may recognize in 

 the interior of these siliceous corpuscles a quadriradiate cross 

 of canals which represent the axes of an equilateral pyramid. 

 The four canals meeting at the centre at 120° are often but 

 short ; they either cease before the first furcation of the arms, 

 or divide by bifurcation and pass for a short distance into the 

 two main branches, without, however, reaching the root-like 

 processes. These canals are usually of capillary fineness ; but 

 sometimes they are considerably enlarged, probably by chemi- 

 cal action during the process of fossilization. In my memoir 

 on GoeloptycMum^ I have figured a number of such corpuscles 



* Denkschr. bavr. Akad. CI. ii. Bd. xii. Taf. vii. fies. 11-15, 20-23, 28, 

 32,33. 



