ORDER I CALCISPONGIAE—PHARETRONES 63 
seen radiating in all directions from numerous centres of crystallisation. —Some- 
times such calcareous skeletons afterwards become silicified. It is clear, 
therefore, that the present chemical composition of a fossil sponge furnishes us 

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MW 
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Fic. 86. Fic. $7. Fic. 88. 
Fibres of a Pharetrone, composed of three- Solid fibres of fossil eal- Fibres of fossil calcareous 
rayed spicules (Peronidella cylindrica, Goldt. careous sponge with partially sponge altered by crystallisa- 
sp. Upper Jurassic), 49/7. preserved spicules, 40/;. tion, 49/;. 
no clue in regard to its original character, since during the process of fossilisation 
a silicions skeleton may become converted into a calcareous, and a calcareous 
into a silicious. 
Of the four orders of calcareous sponges—Pharetrones, Sycones, Ascones, and 
Leweones—only the first two are of practical importance to the palaeontologist, 
traces of the others being either wanting or extremely fragmentary. 
Order 1. PHARETRONES. Zittel. 
Wall 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 

Fic. 89. 
Peronidella - Fic. 90. Fie. 91. 
cylindrica, Mnst. iin E 
sp. Upper Jur- Peronidella dumosa, From. Corynella Quenstedti, Zitt. Coral-Rag ; Natt- 
assic; Muggen- sp. Hils; Berklingen, Braun- heim. «a, Sponge, natural size; b, Skeletal 
dorf. Reduced 1/s. schweig. Natural size. fibres, 4/]. 
round osculum, Dermal layer smooth, perforated by ostia of short canals. 
Triassic and Jurassic. £. clavata, Lamx. ; 
Peronidella, Zitt. (antea Peronella, Zitt. non Gray, Siphonocoelia, Polycoelia, 
