clii 
THE VOYAGE OF H.M.S. CHALLENGER. 
Genus 3. Corticella, n. gen. 
Cortieidse containing polyactinose in addition to tetractinose asters. 
Type — Corticella stelligera (0. Schmidt) (p. 281). 
Genus 4. Rhacliella, n. gen. 
Corticidee (?) characterised by polycladose calthrops. 
Type — Rhacliella complicata (Carter) (p. 281). 
Family III. Thrombida:, Sollas. 
Thromhidx, Sollas, Encyclopaedia Britannica, vol. xxii. p. 428, 1887. 
Microsclerophora with trichotrisenes, and sometimes a peculiar form of amphiaster. 
The ectosome is thin and not sharply defined from the choanosome. The mesoderm is a 
dense collenchyma, containing numerous large granular cells in addition to collencytes. 
The chamber-system is diplodal. 
Genus 1. Thrombus, Sollas. 
Thrombus, Sollas, Sci. Proc. Roy, Dubl. Soc., vol. v. p. 179, 1886. 
With the characters of the family. 
Type — Thrombus challengeri, Sollas (p. 275). 
Great interest attaches to the suborder Microsclerophora ; the family Placinidse 
evidently lies near the root of the Tetractinellida and of the Spintharophorous Monax- 
onida also, while it is not separated by a very wide interval from the still more primitive 
Myxospongise ; were Oscarella lobularis to acquire tetractinose asters it would of necessity 
be included in the Placinid^. The fact that in my preliminary report I included in the 
Placinidge the new genus Epallax, which on fuller consideration I now assign to the 
Axinellidse, shows how nearly these families are related, and how closely the Spintharo- 
phora and Tetractinellida approach each other. 
Order II. LITHISTIDA, 0. Schmidt. 
^ Lithistidx, 0. Schmidt, Spong. Atlant. Gehiet., p. 21, 1870. 
„ Carter, Ann. and Mag. Nat. Hist., ser. 4. vol. xii. p. 437, 1873. 
Subfamily Lithistina, Carter, op. cit., ser. 4, vol. xvi. p. 185, 1875. 
Order Liiliisiidx, Zittel, Abhandl. d. k. baier. Akad. d. Wiss., Bd. i. p. 67, 1878. 
Suborder LitMstina,Yos,ms.e.r, Bronn’s Klass. u. Ord. d. Thierreichs, Porifers/, Bd. ii. p. 281, 1887. 
Tetractinellida provided with a consistent skeleton by the zygosis of modified s]3icules 
or desmas. 
