Prof. W. Thomson on the " Vitreous " Sponges. 119 



Suborder 3. (Halichondrida) Spongina (Lieberkiilin). Ske- 

 leton an elastic wide-meshed network of anastomosing horny 

 fibres, frequently containing foreign bodies, such as grains of 

 sand and spicules of other sponges, and occasionally siliceous 

 spicules developed within them, and never associated with free 

 siliceous spicules in the sponge-mass. Ex. Sjpongia^ Chalina^ 

 Dysidea^ &c. 



Order 3. (P. Silicea) Coeticata (0. Schmidt). Globular, 

 tuberous, or branched Sponges, supported by regular radiating 

 sheaves of long siliceous spicules, and invested with a special 

 dense cortical layer, often containing spicules of special and 

 characteristic forms. Ex. Tethya, Geodia^ Placospongia. 



Order 4. (P. Silicea) Arenosa (= Arenospongia, Gray). 

 " Sponge consisting of a disk of agglutinated sand, with a 

 series of diverging spicules on the circumference of the disk, 

 and a pencil of similar spicules at the mouth of the oscules on 

 the upper surface of the disk." (Gray, I. c.) Ex. Xenospongia. 



Order 5. (P. Silicea) Halisaecina (Lieberkuhn). Sponge 

 an extended sheet of sarcode, destitute of either siliceous or 

 horny support. Ex. Halisarca. 



My principal object in the present communication is to de- 

 fine the first of the siliceous orders, the glassy Sponges. I 

 believe the following to be the scope of the group in known 

 genera and species : — 



Order I. (Porifera Silicea) VITREA. 

 Genus 1. Habeodictyon*, n. g. 



H. speciosum^ Quoy & Gaimard (sp.). 



H. corhicula, Valenciennes (sp.). 

 Genus 2. Hyalonema, Gray (in part). 



H. Sieboldt.) Gray. 



H. lusitanicum^ Gray. 

 Genus 3. Euplectella, Owen. 



E. aspergillwn^ Owenf. 

 Genus 4. Aphrocallistes, Gray. 



A. heatrix^ Gray. 

 Genus 5. Dactylocalyx, Stutchbury. 



D. pumicea^ Stutchb. 



D. sidjglohosa^ Gray. 



D. Prattii, Bowerbank. 



D. callocyathes^ Gray (sp.). 



D. azorica^ Gray (sp.). 



Z).? torva, Duchass. & Michelotti (sp.). 

 Genus 6. Faerea, Bowerbank. 



F. occa^ Bowerbank. 



* d^pos, delicate^ and bUrvov, a net. 



t Through the kindness of Dr. Farre, I have had an opportunity of see- 



