ORDER I 



CALCISPONGIAE— PHARETRONES 



69 



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sometimes they are closely opposed iii the form of solid anastomosing fibres 

 (Pharetrones). Regulär triaxial spicules are of the most common occurrence, 

 next monaxial spicules, sharpened on both sides, and 

 more rarely four-rayed spicules. 



Ovving to the ready solubility of the skeletal Clements 

 in calcareous sponges, they are usually but poorly pre- 

 served in the fossil state, and are ill-adapted for micro- 

 scopical investigation. The three-rayed and rod-shaped 

 spicules which are united in fibres are seldom distinctly 

 recognisable as such, since, as a rule, they are either 

 wholly or partially dissolved, and are converted into 

 homogeneous or cry stalline fibres of calcite (Fig. 84) ; in 

 these minute threads of calcite may be seen radiating 

 in all directions from numerous centres of crystallisation. Sometimes such 

 calcareous skeletons afterwards become silicified. It is clear, therefore, that 

 the present chemical composition of a fossil sponge furnishes us no clue in 

 regard to its original character, since during the process of fossilisation a 



Triaxial skeletal elements 

 of a Recent Ascon, ^/i. 



Fio. 82, 



Fibres of a Pharetrone, composed of three- Solid fibres of fossil calcar- Fibres of fossil calcareous 

 rayed spicules. Feronidella cylindrica (Gold- eous sponge with partially sponge altered by crystallisa- 

 fuss). Upper Jurassic, 40/j. preserved spicules, ^o/^. tion, 40/^. 



siliceous skeleton may become converted into a calcareous, and a calcareous 

 into a siliceous. 



Of the four Orders of calcareous sponges — Pharetrones, Sycones, Ascones, and 

 Leucones — only the first two are of practical importance to the paleontologist, 

 traces of the others being either wanting or extremely fragmentary. 



Order 1. PHARETRONES Zittel. 



^rall thick ; canal System like that of the Lithistida, though sometimes indistinct 

 and apparently absent. Spicules arranged in solid anastomosing fibres ; a smooth or 

 corrugated dermal layer frequently present. Devonian to Cretaceous ; unknown in 

 Tertiary and Recent. 



Eudea Lamx. Cylindrical or club-shaped, usually simple, rarely branching. 

 Cloaca narrow, tubiform, extending to the base, and terminating above in a 

 round osculum. Dermal layer smooth, perforated by ostia of short canals. 

 Triassic and Jurassic. E, clavata Lamx. 



Peronidella Zitt. (Peronella Zitt. non Gray ; Siphonocoelia, Polycoelia From.), 

 (Figs. 85, 86). Thick - walled, cylindrical, simple or branching. Cloaca 

 tubiform, extending to the base ; base sometimes covered by a dense dermal 

 layer. The rest of the exterior finely perforate. A distinct canal-system 



