Hexactinellidae and Lithistidae. 465 



Loc. Western entrance of the English Channel ; and the 

 sea about the Seychelles. 



Obs. This exquisite little arabesque structure (PI. XVII. 

 fig. 7, b) 1 from its vitreous appearance, and from not dissolving 

 when boiled in nitric acid, together with the form of its frond- 

 like branched and filigreed spicules (fig. 8, a a), seems to belong 

 to the Lithistidse ; but, like the preceding species of Farrea 

 densa as well as F. occa, it has yet to be found in a living state 

 for this identification, and for the remaining part of its descrip- 

 tion. While some portions are found on the vitreous fibre of 

 the sponge mentioned, others are observed to be separated 

 from it (fig. 9), as if the extent to which the structure had 

 grown round the spicule and had formed by union a continuous 

 sheath had determined this. It appears to be the product of 

 a creeping sarcode, like that of the Spongiadas ; and therefore 

 I assume for the present that it is the structure of a sponge. 





EXPLANATION OF THE PLATES. 



Plate XIII. 



Hexactinellid^;. 



Skeleton -spicule. 



Fig. 1. Three small skeleton-spicules, showing the way in which they 

 are united to the main fibre and to each other to form the 

 skeleton-structure : a, fragment of main fibre ; b, small skeleton- 

 spicule united by one arm to the fibre ; c c, skeleton-spicules 

 united by one arm to the spicule b. From Aphrocallistes Bo- 

 cagei. 



Flesh-spicules. 



Fig. 2. Eosette with long arms and short, straight, pointed, dual rays : 



* acta acta, arms ; bbbbbb, rays. From Crateromorpha Meyeri, 



Gray. 



N.B. After this the fifth and sixth arms (c c), or third axis, 

 will, for the sake of perspicuity, be omitted. 

 Fig. 3. Rosette with short arms and long, straight, pointed rays. Eu- 



plectella aspergillum. 

 Fig. 4. Rosette with three-rayed arm. Euplectellidce. 

 Fig. 5. Rosette with two-rayed arm ; rays straight, capitate, few- and 

 long-spined : a, caput or head ; b, head, more magnified, to show 

 spines, end view ; c, the same, lateral view. Rossella velata, 

 Wy. Thomson. 

 Fig. 6. Rosette with short arms and many straight capitate rays : a, many- 

 spined head, end view ; b, the same, lateral view. Dactylocalyx 

 subglobosa, Gray. 



N.B. This is the usual form of head, although the spines may 

 not always be distinguishable except with a very high power. 

 Fig. 7. Rosette with long arms and many straight capitate rays. Dacty- 

 localyx subglobosa, Gray. 



